The route from Mazalat hut to Eho (or Echo) hut follows the main ridge of the Central Balkan mountains in Bulgaria. It is a part of the 600 km long E3 route and is actually the most difficult section of it. Some of the highest peaks are concentrated in this area, dominated by Botev peak (2376m).
The original idea dates back to 2002-2003 when a group of enthusiasts did the route and called it "StaRa 100x24" or 100 km in 24 hours in Stara Planina (the name of the Balkan mountains in Bulgarian language). Later it turned out that the total distance is "only 68-70 km" but the name remained unchanged.
Originally, as part of the E3 route, most people do it from west to east, because it's a tradition to start from the westernmost point (Kom peak, 2016 m) and finish on the Black sea (cape Emine). The reason that 100x24 is done in the reverse direction is that the most difficult places are in the first 35km and most of the people do it during the daylight, with an early start from Mazalat hut.
Best time of the year to attempt the route is between early June and late October, depending on the snow conditions. Water and supplies are available in several huts along the way and usually this is not a serious problem. In a hot summer day in July-August dehydration is not to be underestimated. Big problems can occur in bad weather as some parts of the ridge are quite large and covered in grass, mixed with steep, narrow, rocky parts. Fog is very likely to cover the highest peaks and route knowledge is highly recommended.