I raced the Iowa MTB race series marathon events the prior two years and what Rage said is spot on, eat a deece safe breakfast and run a Camelbak full of water and nutrition either in a bottle or in a pocket or bag and try to determine how and what to eat at what time interval but ~ 4 hrs shouldn't be that bad. I would say butt on saddle time is biggest thing, I never rode full race distances before race usually like if I do 75% of distance in practice I know I can full distance. Take a little break for those legs to rest week before you race and otherwise you got it. I have done 4 hr races on hardtail and FS and the FS even if a "less race efficient" bike is gonna keep your mind sane longer to put in more work. Biggest pointer is to not run out of water, I drained a Camelbak one race (still had nutrition and hydration and a trailside can of Coke so not like I would actually die!) but the lack of plain water headcased me so bad... keep it flowing!