I should have looked at the red print and listened to Sheldon Brown before I set out on my new bike over the weekend:

If you are coming off of a layoff of months or years, start with very short rides, maybe a mile or two, no more. Only gradually should you increase your ride distance. This may seem frustrating, but it does take a while to re-accustom your derrière to cycling. Anybody in decent shape can hop on a bike and ride 15-20 miles, but you’ll be a wreck afterwards if you haven’t accustomed your body to cycling first.

So. Much. Pain.