Mountian Bike Training Tips

I am new to mountian bike racing (not competed in a race yet) and would appreciate any training and or racing tips. I watched a local race a couple of weeks ago and noticed that I am a bit bulky compared to the elite racers. I am 6'0 and 190 pounds with 12% body fat. Is a larger muscular build going to slow me down on climbs and across the flats? Is it true given equal conditioning that smaller guys tend to climb better and larger guys do better on down hill sections? Is that just simple laws of physics? It looks like most of the races are a mixture of xc and tech single track the finish times are 50 to 70 mins. I noticed most of the guys ride hard tails with a few xc f/s in the mix. I ride a 27 speed Rock Hopper Comp which I think will work for the beginner class. I am riding 4 days a week around 20 miles each day. Usually I ride a few miles (3 to 6) on the road to warm up then real hard in trails which are considered difficult for around 50 mins and then a few miles again spinning at the end. Please let me know any training tips you fellow riders might be kind enough to share. Thanks, 🙂