for maximum race performance you need to "taper" your training (the terms more used in running, but still applies for cycling).
basically, the benefits from training come 2 days to 2 weeks AFTER training, so training you do on Friday and Saturday will HURT more than help your race on Sunday. when you train for improvement, you actually break down your body and your muscles and then it is rebuilt stronger. Thus, maintenance training that doesn't break down the body is OK (but won't bring any new improvements) but intensive performance-improving training should NOT be done in the days before a race if you want maximum performance.
there are various recommendations from different trainers and experts... from not riding at all to training at lower intensity to training less time... i fall into the less, very low intensity exercise if you feel OK, and otherwise SKIP it!
what i personally do:
* with a Sunday race, the week of the race do most of your cycling Wednesday or before (and ALL of your intensive training before Wednesday)
* then Thur/Fri/Sat do one short (say 3/4 to 1 hr) LIGHT SPINNING session on the bike - or work on technical stuff like wheelies and hops or whatever. if you don't feel 100%, skip the training. it's really only to get the blood moving and keep the body "ready" to ride. most important is take it easy. if someone passes you DON'T sprint to catch up - RIDE SLOW. NO BIG GEARS!
* then, since you run too (i train about same bike/run breakdown as you), do a run Friday or Saturday -- if Saturday then not longer than 45 minutes (or the distance that makes you tired the next day). the cardio system can usually recover more quickly than the muscles, so some cardio training Fri/Sat usually won't hurt much
for the 3 days before the race, the key is to NOT stress your body so that it is weakened and needs recovery. thus, so less intensity and less duration than your normal training.
IF it's a really big race, then you should have trained really hard for the 3 weeks preceeding the week of the race and then train very light the week of the race. you loose almost nothing in training in 1 week and are then recovered to your max. i.e. all the training benefits without and time deteriation from 1 week PLUS high level of recovery. of course the next week after the race you will notice performance decrease from less training (plus the break-down from high exertion in the race)---- i.e. a small 2-4 day weekly cycle and a large 3-4 week cycle for intensive/light training. then, for pros, there is also an even larger near-annual cycle so that pros plan to reach peak performance for one weekend of the year for the most important race.
Mike, good luck with your first XC race!! also don't forget to "carbo-load" the night before, eat a good breakfast 2-3 hours before and start drinking a lot of water a few hours before the race to start well-hydrated.
P.S. i finished 138 out of 800 (time 3:27 -- first place pro rider finished in 2:32) in the Oberstdorf (Germany) MTB Marathonon (long XC race) September 29 with 55km and 1600 meters of vertical climbing... i had a good race except i crashed in a muddy rock garden and banged up my shoulder, hip and knee but i got back on the bike and finished...