I went on a quick 10 mile ride today after work. It has been a long time since I have ridden much and I've been getting kind of fat since I graduated from college, so there is more of me to pull. I averaged about 17mph or so on the ride over fairly flat terrain, so I wasn't pushing myself hard really, just moving steadily. In fact, I wasn't ever even really breathing hard. However, my back was getting really sore as I was riding. Is this a normal phenomenon when you're out of shape, and will it go away as my back gets stronger? I think it could also possibly be due to a newly acquired bike with different geometry from the last, but I can't be sure. I've gone on longer rides before when being relatively out of shape (but not as much as now), but don't remember my back getting this sore.