"At this point, my fitness goal is simply 3-4 intense one-hour sessions per week. The most important thing is finding a bike that I love to ride. Comfort, quickness, and weight are probably my top three considerations."
Don't rule out road bikes. I hadn't ridden a road bike in 25 years when I test rode one in 2006. They really are comfortable now. Get some good tires in 25c, like Continental GP4000, after the cheaper stock tires are worn down. (Or get the LBS to change them out at purchase)
I found I really like group rides. They push me harder than I would do by myself. I have to make sure I ride a few times a week to be able to keep up with the groups, so it's good positive feedback and motivation.