There's a blog on Garmin's Motion Based site that contains tips and tricks along the lines of:
We hear it quite frequently, "I've got this ultra marathon and I want to track it with my Forerunner 305. How do I go about making sure I capture all the data?"
I generally find that 8 to 10 hours of battery life is enough for one session, but then I'm not entering 100 mile races .... yet.
I also see that Garmin are sponsoring Tim Don, Great Britain's World Champion Triathlete.
Now he's using MotionBased for performance analysis, I can see (at tim.don.motionbased.com) how much faster he moves than me (at nickbrowne.motionbased.com).
Always nice to get a reality check of a Saturday morning.