Fastpacking is a great way to visit Nepal… you carry a little bit of load in your (fast-) pack, but you can use all the facilities of tea-house infrastructure that Nepal is famous for. Then, slightly easier, come running camps and running tours. They can be anything from teahouses and short days, with gear carried for you, to super luxury trips with gentle runs in the nicest of locations in Nepal.
You can search this site for a list of previous fastpacking adventures in Nepal here, and also there is a useful article with 10 tips for fastpacking in Nepal including information about routes, weather, food and hygiene and equipment selection. There is another article for fastpacking equipment selection here.
See also: all events, single day ultra trail races, stage races or multi-day trail races in Nepal, fastpacking trips, 100 miles races in Nepal, and finally trail marathons in Nepal.