The hardest part for me about running, or any exercise for that matter, is willfully putting myself in a state of discomfort. If there was a magical machine that could stress my muscles for me, then I don't think I'd mind the puffing, sweating and thumping heart pain. It might violate the Geneva Convention, but it would be easier to exercise if I were able to abdicate responsibility for continuing.
I guess that's part of what makes a top-tier athlete: the ability to push past limits and train yourself to go past limits that stop everyone else.
That's probably what makes for a top-tier anything.