Funny, I had the opposite experience today. I've been a fairly avid mountain biker since I was in my teen's in the mid 90's, except for the past couple years i've been almost exclusively riding my road bike and commuter bike. I've been road biking a lot the past several months ~150-200 miles per week. Suddenly this week I had a desire to do some mountain biking so I set up the old rigid mountain bike i've been commuting on for trail riding. I removed the drop bars I had on it and the slicks, put on flat bars and a set of 2.1 x 26" knobby tires and I was ready to go.
I can't believe how much fun I was having pounding the single track on that rigid bike today, and I was wondering why I haven't been on the trails the past couple years. I felt like I was flying over the roots, rocks and everything else on the downhills, and because I've been road biking a lot I had plenty of strength to power up short steep sections. Of course I was riding by myself and so I had no one leaving me in the dust to bring me down, but I really think it's possible to have a ton of fun on a fully rigid bike and you can go pretty fast too. Maybe my viewpoint is also different because I was a teenager in the 90's and the only mountain bikes I could afford were the used, full rigid ones from the early 90's and late 80's so to me riding rigid seems pretty normal.
Just have fun with your bike!