[WalterBright]

Running certainly started out painful and boring for me. But after a year, things changed. I started to look forward to it, and it ceased being boring.

I do my best thinking while running, and make use of it to solve problems, write articles, etc.

Lifting weights, however, remains painful and boring :-( and I think that is because it requires mental focus. Running does not, you can just drift off into something that interests you.