I’m a park rat. I have one bike, a YT Capra. But my Dad used to race road bikes, but knows nothing about mountain biking. He wants me to join in a 15 mile race with a bunch of xc riders. Not only will I look probably look stupid, I will probably get left in the dust. Now, I can pedal myself up hills no problem, and I could finish the race eventually, but it would be a lot more inefficient with an almost 7 inch travel bike.
So it is a XC mountain bike race? If you are in a good shape, cliped in you could still come in the midddle of the pack on a bike with bigger travel because 15 miles is a short race. If it would be lets say 60km then there is a higher chance of getting droped by the guys with shorter travel bikes. You will be ok doing 15 miles as long as you are willing to push yourself and have a good cardio.