Distance makes little sense to me as a challenge, at least not without also a speed requirement. Once you get beyond a certain point of fitness running a marathon is trivial unless you care about your pace. To challenge myself in distance I would need to run more hours than I think is fun (12+ hours), and honestly that is more of a time challenge than a distance challenge.

So the reason I go for speed is that it is a good way to challenge myself. Plus running fast is fun.