Go for it LivingWater! How long will that fast be?
When i was in to about day 16 of my 20 day JF, i HAD to go hiking and running some 6 milers, just to get rid of some of the energy. ;-) Then i got to thinking about something like you're going to do. Why not hike the Chilkoot Trail in Alaska, a 34 mile hike while fasting.
I did a marathon, 26.2, many years ago, and if you can run a 13 mile, you can do a marathon...that's the longest i'd ever run, 13 miles, about 4 times, then did the marathon. And that was withOUT all the energy from a nice fast.
Keep us posted how it comes out. (I've got a tip or two on running the big one, if you're interested)