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Be sure to check Hiking in Europe page for additional information and links.
As of 2018, Bulgaria is not part of Schengen. Thus Americans can spend 3 months in the Schengen zone plus 3 months in Bulgaria.
I hiked western half (west of Dobrila hut) in 2017 (see here) and eastern half in 2018 (see here) in 27 days total not counting rest days. GPX track mostly valid, though tends to remain on zigzagging roads rather than following more direct foot trail. Trail normally well marked, so follow waymarks in preference to GPX where they differ, especially in well-maintained central section. Bgmountains.org map shows almost completely accurate E3, while OpenAndroMaps map shows mostly accurate E3 (more about maps below).
About 30km east of Kotel, GPX makes long detour along paved roads (thorough Byala Reka and Rish) to get around heavily overgrown sections of official trail. I was able to get through official trail in 2018, but it could easily become impassable if blackberry thorns allowed to grow much more. Bgmountains.org map very helpful with this overgrown section, because it shows alternate paths and forest roads not shown on other maps.
In west and east sections of E3, mountain huts shown on maps may not be open or may have very limited food, so be prepared to carry 3 days food, 5 liters water, 5 days power for electronics. 5L water carrying capacity only needed if dry camping (away from water source) and then only for short distances, otherwise 3L should be sufficient. E3 guidebook available here and also at some bookstores in Sofia.
Lift Sopot is cable chair lift connecting lower station at 590m in town of Sopot with upper station at 1390m (800m ascent/descent), just below Dobrila Hut at 1790m. Sopot has frequent bus connections to Karlovo, which has frequent connections to Plovdiv. Dobrila hut near midpoint of E3, thus good place to take rest break and resupply with trail food from grocery store.
I hiked in 2017 (see here) in 16 days not counting rest days. GPX track valid other than trivial instances where it shows straight trail rather than switchbacks. Follow E4 waymarks in these cases. At south end of Rila mountains, after Mt Kapatnik, I followed first well-used dirt road I encountered to get Razlog (this detour included in GPX file above) rather than descending along E4 to highway, so I can't confirm GPX track for this section. Another hiker who did descend directly to highway reported trail there was overgrown and poorly marked, so that he ended up bushwacking his way down.
To get to start of E4, take Sofia metro to Vitosha station, then bus 93 to Dragalevtsi ski lift. On Saturday and Sunday, bus 66 goes from Vitosha metro station to ski resort at top of ski lift, cutting out 1000m of ascent. Bus information current as of August 2018. To get to ski resort weekdays, without walking up, hire a taxi. Path to ski station shaded and not bad when mild temperatures in Sofia, but best avoided when very warm.
GPX (as of Oct 2018), Google Maps. Best time of year to hike: September/October. 535km for section from Samokov to Greek border. (72km section between Serbian border and Sofia is lowland road walking. 71km section from Sofia to Samokov mostly coincides with E4.)
E8 was planned to run west to east through Rhodope mountains from Samokov to Greek border, but little information available as of 2018, other than overview sketches. Sultan's Trail is alternative, evidently based on planned route of E8, with occasional white/red/white E8 waymarks and E8 signs dated 2004. Official website includes current GPX track, resupply and accommodation info, trip reports.
I hiked in 2018 (see here), in 24 days, not counting rest days. I had some problems with GPX track available from Sultan's trail website at that time (problems may have been fixed since then, notes below assume west to east direction of travel):
Locus mapping GPS app for Android discussed on GPS page. LoMap map of Bulgaria, available in Locus map library, is high quality, as is map at FreizeitKarte-OSM.de, but map at OpenAndroMaps.org is best, in my opinion. For external maps, be sure to download both map and map theme/style. All these maps are offline, meaning downloaded to smartphone in advance and do not require internet connection for use.
OruxMaps another mapping GPS app for Android. Does not have built-in map library, but does support both OpenAndroMaps and FreizeitKarte-OSM maps and map themes/styles. Can also download unlocked Garmin IMG files (such as map at bgmountains.org) and place into OruxMaps mapfiles directory, as described in OruxMaps user's guide (available at OruxMaps website).
I found Locus easier to use than OruxMaps, and so mostly used Locus with OpenAndroMaps map during my 2018 Bulgaria hiking trip, though I sometimes used OruxMaps for access to the IMG file map at bgmountains.org, which is the most detailed and complete map of Bulgaria hiking trails, water sources, etc, though somewhat cluttered looking.
GaiaGPS mapping GPS app runs on both Android and IPhone. Builtin Gaia topo map is adequate, if combined with GPX track. Can also install bgmountains.org map using one of following methods: (a) logon to GaiaGPS.com and add this URL to your list of map sources; (b) download this TileJSON file, then upload to GaiaGPS.com to create new private map source. Read help files on GaiaGPS website for how to add/create custom map sources. In either case, map source "Bulgaria Topo Map" will appear in list of map sources for GaiaGPS on your smartphone. Download map data to smartphone in advance for offline use, same as with other GaiaGPS map sources. I used GaiaGPS during my 2017 Bulgaria hiking trip, but now prefer and recommend Locus and OruxMaps apps.
I have heard or read good things about Viewranger, MotionX, and Backcountry Navigator smartphone apps. However, I have not used any of these apps myself, nor do I know what maps are available for these apps. Someone also recommended Soviet Military Maps app.
For all maps, whether electronic or paper, paved road information usually accurate, but forest road and path information less reliable, for the usual reasons: sometimes more or less roads/paths in actuality than shown on map, roads/paths may be overgrown or abandoned or blocked by fences, roads/paths may be routed differently from what is shown on map. Even Google Earth cannot be fully trusted, since satellite photos cannot show thin barbed wire fences with "do not enter" signs blocking roads. On other side of such fences may be owners hostile to trespassers, dangerous loose bulls, packs of vicious dogs, etc.
Based on my experiences in 2017 and 2018, English widely spoken by people under age 30, and Russian widely spoken by people age 50 and above. Also, even those Bulgarians who never studied Russian can still understand it somewhat, given the large number of words in common between the two languages. Also, anyone who reads Russian can parse written Bulgarian, so as to get the gist of what is being communicated. Given lack of learning materials for Bulgarian and lack of usefulness of Bulgarian outside Bulgaria, in my opinion better to learn Russian rather than Bulgarian, for English speaking tourists who plan to learn any foreign language for use in Bulgaria.
However, even if you speak both English and Russian, you'll still need a dozen or so Bulgarian phrases/words for people who only speak that language (yes, no, please, thank you, there are, there are not, are there available rooms in hotel, do you speak English, I only speak English). Be sure to download Google Translate offline file for Bulgarian, in case no internet connection available for online translation.
A1 mobile (formerly Mtel) is oldest mobile network operator and probably has best coverage in rural areas. Probably most expensive service, however price difference between operators is small. A1/Mtel has large store inside Mall of Sofia in central Sofia as of 2018. Be sure to download A1 mobile app to allow checking account balance and purchasing add-on packages. Switch from Bulgarian to English version of app using settings menu. As of September 2018, it appears initial SIM purchase includes generous data and minutes allowance, valid for 30 days, but you also need some account balance for SMS messages. Each time you add 10 lev, system deducts 5 lev for 2GB data and 100 minutes calling, valid for 30 days. If that isn't enough, first add-on under "Null" category offers 3GB data and 150 minutes calling, valid for 30 days, for 15 lev. Bulgaria country telephone code +359.
Booking.com useful for finding hotels. Sometimes booking.com shows vacancy in small hotel but either hotel is closed or no vacancy.
Rooms.bg also useful for finding hotels and rooms/apartments for rent, especially in small towns not well covered by booking.com, but as of 2017 it doesn't show room availability or allow for making online reservations, plus site in Bulgarian language only, though mobile version not difficult to navigate.
Airbnb.com another possibility for finding lodging.
In my experience, ticks a big problem at lower elevations in June, but not a problem at lower elevations in September/October, or higher elevations in August/September. No personal experience with other months. Ticks in Bulgaria known to carry tick-borne encephalitis and other dangerous diseases.
Walking in Bulgaria's National Parks by Julian Perry (2014), available on Kindle. Same author wrote Mountains of Bulgaria (1995), which is out-of-print but useful, since it contains full end-to-end guides for E3 and E4 trails, whereas newer book only covers shorter hikes in national parks. Balkan Trek is guide company run by Julian Perry.
BREAKFAST (omelets often served all day)
Some hikers eat only at huts, but I usually carry food, for snacks during rest stops between huts and for situations where huts are far apart and so camping required. As elsewhere in Europe, my trail foods are: bread, hard or at least semi-hard cheese, roasted peanuts, dark chocolate. In warm weather, I eliminate cheese and chocolate and so only carry bread and peanuts. Bread can be purchased at huts, but peanuts (фъстъци, фъстък = single peanut) are seldom available, so good idea to stock up during town stops.