I used to be a competitive cyclist, and I keep checking up on race results, etc.
There's a 10 day off-road race in Australia called the Crocodile trophy. Mixture of fire roads, gravel roads, 'real' offroad, etc through the Australian outback. 1234 kilometers or 765 miles in 10 days. Brutal temperatures, dust, and hills.
This guy did it on a hand cycle:
After another recent thread about human health struggles, I'll never feel sorry for myself again.