i've been using the hope HIDs recently, pretty bright, long narrow beam, best used with a headlight for more technical trails due to the narrow beam. infact I'd recommend both a bar mount & headmount for better depth perception, undulations in the trails, sharp corners & long grass/reeds/ferns
I'd start with trails you know well before going further afield. also bring a small light incase of mechanicals. I also bring a spare battery, aside from getting a 5hr burn time, not really worth the risk of running outta light miles from anywhere! battery life is normally less when its very cold, so factor that into the burn time.