At my local mtb center I regularly do a 30 mile run. The trails are a mix of single and double track - your average Ontario Cup racing loops (actually 2 loops).
Here is my food/hydration prep:
1) The day before I eat mostly carbs, but no more than about 2500 calories
2) On ride day I have 1-2 bannanas about 1 hour before starting the ride
3) During the ride I will consume about 2 liters of water (which is what my Camel Back can carry) and 3 gel packs. One gel pack before starting and then a gel pack after each hour of riding.
4) I ride non-stop and it takes me about 3 hours and 40 minutes, give or take a couple of minutes depending on conditions.
I am in decent shape (I can run 20 miles in about 3:15 and plan on entering a marathon in September) yet elite riders can do this distance in around 2:40, a full hour better than I can. Kinda makes me wonder what WTF are those guys made of?