You, people, you don't trust Rainbow's skill at all. I feel ashamed for all of you! Nah. Personally, I go with Rainbow Dash, given that she is more of a professional athlete than Applejack; AJ also has what it takes, but she lacks the natural speed and light weight of a pegasus. Their endurance must be quite similar - they both excercise daily. I can say without fear of favour that, if anyone, then Dash would have won. I'd rather not choose, though. Okay, I might be a tiny bit biased. I don't particularly believe that long-distance running's primary requirement would be a large amount of muscle, and I highly doubt that having such would make a difference, given that they are both sporty enough. I base that on what I know about humans and my personal experience, so feel free to correct me. About her stamina - I think she (the one to create a Sonic Rainboom) has trained enough to be able to stand her ground against anypony stamina-wise. They were both running till the end, after all. As of now, it seems I'm alone with my opinion. And yet again, I wrote a book-sized comment. o.o