Guys, I need your help

I've got a trail bike ride coming up that I'm planning for. It's on a four wheel drive sort of road/track where the average speed would be 25 to 30mph and quite a number of slower sections, so no top gear but also not much full throttle stuff either . Looking on the map it's a bit hard to figure out exactly what distance the longest section is, as they don't show all the twists and curves. But I'm thinking it's around 120 miles (200 kilometers). On a stock Honda XR 400R how much fuel will it take to complete this section?
If I survive it

I'll post pic's.