Garmin Forerunner 610 for long runs/hikes (keeping it charged)

So just got back from hiking the Grand Canyon (rim to rim).  It took 10 hours.  Unforunately, my forerunner didnt have enough juice to last through the hike back up.  Its a shame, it was very good about keeping a gps signal 90% of the way.

I noticed on the garmin site there is an external power pack, and it shows the 610 as a compatiable device.  I was wondering if anyone has used this, or some other means to keep the forerunner charged up for longer hikes.  Any ideas?

https://buy.garmin.com/shop/shop.do?pID=86599