Wow, what an interesting experience! Just because someone has good road riding fitness doesn't mean they have good mtn bike fitness. I have not suffered in a road race like I did yesterday in the mtn bike race ever. That's right, I said (or wrote rather), EVER. I wanted to quit several times.
However, due to many amazing people cheering for me and wishing me on, I was pushed and challenged outside my comfort zone. I surprised myself several times and I suffered to the finish. I did not get a good result, but I had a great training day both mentally and physically, and got a new appreciation for mountain bikers. It's a dang tough sport and it almost killed me yesterday.