why does biking make my bum hurt so much?

I took my new wheels out for their first long ride - 100km along the Hudson Hope Road. The first half was beautiful. It was sunny and the riding seemed easy. Then I turned around and realized why the riding was so easy: I had a 20kph tail wind. Which meant that on the ride back, I had a 20kph headwind. Ugg. Still, it felt awesome to be out getting some exercise and training for my big bike trip this winter. I just have to do that, oh, 50 more times and I'll be in Mexico :)