train hard on the road bike and it will surprise you. I hadn't been in the dirt for a month or more and went to canal and was pulling off sub hour loops and was faster than the spring race. It's not like Canal or N/S is super technical. When we raced last year, I was 8 weeks post op from leg surgery. I had been on the MTB maybe 3 times before the race. Maybe that explains my tree smack about 5 miles into it......

Hill intervals on the road bike will help you on the climbs at N/S. That's where it's going to hurt you. You ARE going up in class right so that means 25 miles and some nasty climbs.

I went 5" travel this year and when we prerode N/S on Sunday I took my rear shock out of propedal and went full open for the rough stuff. Really smoothed out the trail and maybe going a softer setting on the rear shock will help your back. My back isn't the greatest either and I'm pretty much hanging up the HT forever. I won the spring race on the HT and was miserable for two weeks after.

You do have some guys to ride road with don't you? drafting and pack riding isn't a big deal if you are used to drafting and passing during a MTB race.