training questions- help please!

Ive pretty much resigned myself to the idea that my MTB season is going to just be a training season for 'cross, but I had some questions about focus. Im thinking about doing some endurance events, 5-6 hour variety and was wondering if this was a good move to build a big base for cross, or should I just focus on XC races and go for the intensity. Im figuring I can ramp up intensity starting in August to get the short-interval stuff in, but Im not sure which would be a bigger benefit between now and then- 90 minutes of MTB at an XC race effort, or dial back intensity a bit and go 4-6 hours at an endurance-race effort? Any help from the gurus would be much appreciated. Thanks!

Step 1: Go to your local Beer Distributor, Pick up a Case of PBR
Step 2: Return Home
Step 3: Drink
Step 4: Sleep off the hangover
Step 5: Return to Step 1 and repeat.

Or listen to someone who really races... I'm not good at real racing advice.