I find that it helps to smile when I feel like my legs and breath are giving out.

One time last summer I spent most of the day hang-gliding- which is strenuous enough. I flew for an hour and half in some nice thermals but by the time I had the glider back on the truck and headed home I was pretty beat. I loaded everything on the garage, which is not at my house and hopped on the bike to head home. Only, I decided to ride to my GF's house which is 13 miles away. I might have made the best time there ever, this on a day where I was already spent. But it was fun and I really got into it and despite some serious hills, could have easily done it again when I got there. Attitude.