I have a Giant XTC 29er as well. I like mine a lot and I've had it for years. Movin on, the gym work idea is a great suggestion. You need to get your body ready to start doing physically stressful activity. You can run, swim, and lifting weights will also help. Your core is very important. Without it, on longer rides you will put too much weight on your hands, which can cause numbness. Also when I start to get tired, I start to lose my form, which causes me more problems.
If you're not big into gym work, you can take your mountain bike out on the clear roads. It will be colder riding on the road instead of mountain biking, but it will help you when it comes time to get out on the trail. I recommend riding on the road or a forest service road instead of jumping head first into mountain biking. You can work on your bike handling skills, going over obstacles, shifting your weight, and all of that important stuff before you run into those obstacles on the trail. Last but not least, make sure to have fun.