Good advice on the saddle, and if you continue getting into the riding, you'll love a road bike someday, like rfenst said. I use a selle italia flite saddle, it's small and it takes about a week of riding with a sore ass, but once you have the seat time all is good. Brooks is supposed to be great, and I think they've been around for about as long as bikes.
Mountain biking though was such a blast. I remember I felt like a little kid again blasting down trails, climbing stuff that I wasn't even sure I could walk up when I first saw it. Groups are good for mountain biking, to help learn technique. Also for someone to call your next of kin, after the inevitable face plant, broken collarbone, etc. Groups good for roadie if you want to go fast for long distances, learn to draft and lead. But alone is also great just to clear the head, and you'll get in great shape. Be careful when you shave your legs though...
I need to stop writing, and go ride. Glad you brought it up, time to dust of the bike for a weekend ride.