I'm not an avid hiker but my mom is, so over winter holidays we go to one of the national parks and do some hiking. We always do at least one intense day-long hike. I can pull it off but I'll be pretty tired at night and sore the next day. But that's the privilege of a normally functioning body, you can pull out some extra strength from your reserves on the fly. Doesn't work that way for fatties, so you can never ask them to come along for something like that.