Hmm... if you're doing a proper XC ride with trees and stuff you're gonna need a proper bright light, not a "don't run me over Mr Car Driver" torch. You really need to see where you're going, and what the ground is like. Off-road it's definitely more about seeing than being seen. I've literally just come back from a night ride round Swinley forest. Helmet mounted lights are a must if you're riding twisty singletrack because bar mounted lights only point where you are currently going, not where you want to go. Those are 2 very different things when it gets twisty!
I use Lumicycle's halogen helmet lights - 12W spot is good enough, an extra 20W flood helps with peripheral vision, but that chews up the Li-Ion battery. Using primarily the 12W spot, and flicking the flood on when it gets twisty, I go through a full charge in under 3 hours though - and that's with the big battery.
I know I'm old tech here though, apparently the very latest Super-bright LED lights can be bright enough and deliver decent run times. Not sure how those 100 lumens stack up against an over-volted 12W spot halogen... I wouldn't want to see any less than I do now, but if Debian or anyone can confirm that these Lenser things really do give enough light to ride singletrack at night I want some! The quoted burn times seems amazing. Actually for £30 I might just get one to find out...
Edit: just done some research on the Lumicycle site. Apparently my 12W flood pumps out about 370 lumens. Don't think I'd fancy my chances with a 100 lumen torch after all, though I might get one for emergencies...
Halogen
http://www.lumicycle.com/product/104/halogen_1220_pro/halogen-1220-off-road-pro.html
HID
http://www.lumicycle.com/product/100/hid-bike-light-------700-lumen.htm
LED
http://www.lumicycle.com/product/204/850-lumen-flood.htm
Judging by this I'd probably say that to get 10 hrs run time with enough light to ride by you'd be looking at £250! (It's the battery that costs all the money).
BUT check this out: Lumicycle will hire you the kit, so you don't need to invest loads of money on something that you might not use. Mind you, it could still cost £50-60 to hire it for a weekend.
http://www.lumicycle.com/product/173/hire.htm