Assuming your hare scrambles in your area are like the ones here in the midwest, the course is usually 4-10 miles long, lap times are typically around 10-25 minutes (depending on the course length and your skill level). So in the 2 hours, you will usually do several laps.
If your tank can't make the full 2 hours, you can leave a gas can somewhere by the course and the pit area, and stop and top off. You will usually see a place that other riders leave there cans, usually by a spectator area.
And if you can't finish the full 2 hours, you can just pull off the course at a checkpoint near the pit area, after completing however many laps you want to finish. If they use fender cards to score you, just make sure to turn in your fender card if you pull off early.
For water, you could also leave that by your gas can. Lots of riders also wear Camelbacks, I love mine for hare scrambles!