I run ultras with an old Garmin 405 (battery life now 6hrs max) and a small Duracell USB battery pack. You charge it up from your PC and then plug your Garmin charging clip into it. Works great! It's small enough to hold in your hand while running and re-charge on the go. Only drawback is that for the 45mins that the Garmin's charging, you can't read the on-screen bumf about time and pace etc, as the screen just shows 'Charging' but as I'm using it mostly to just record my stats and not to actually pace myself by them, it works fine for me.I was thinking of splashing out on a 910xt but this option only cost me £7 instead of £350+. Just mentioning as you said you already have a Garmin...
A fellow ultra runner mentioned something to me t'other day about one of the Suunto watches that only takes a GPS fix every minute, so has a GPS battery life of days. Only gives average pace not current pace, but that's probably enough for most ultra people.
Edited: 30/07/2012 at 18:13