As far as I am concerend if I enjoy what I do, then I don't coinsider it work.
Doing something for a reward, in whichever form it comes, is, per definition, work. You just don't consider it as such. You are the one that defines if it's work or fun.
And as for your running analogy, I'm not sure how relevant that is.
Same difference. It is either a horrible ordeal or a great fun. However you consider it.