Honestly I think it's that it just starts to suck less with each time you do it. I've run the gamut in my life from overweight couch potato that didn't exercise for years, to fanatically going to the gym 3 hours a day, five days a week. Running an 8-9 minute mile when I was in my worst shape would have been orders of magnitude harder for me, mentally, to complete - than probably a half marathon or something when I was in good shape. That doesn't mean I've _ever_ liked going to the gym and working out. It did mean that at times when I was in such good shape I didn't mind it that much.