[suicideneil]

What about running a dynamo in conjunction with the motor on a stock emaxx motor mount, then use that to recharge the batteries as you drive! You would need a small bicycle dynamo, and a really torquey motor, but it might extend run times by a little.
I was actually doing some more late-night research into batteries, and I saw the specs on some D-cell NIMH.... up to 9000mah. Granted, they would weigh nearly twice as much as sub-c cells, but think of the runtimes....
Going the other route, I saw some good AA-size Nimh with about 2500mah capacity @ half the weight of sub-c. Now, you could build two 14.4-16.8 packs, that wouldnt be much larger than 2 7-cell packs, and then run them in parallel to get 5000mah+ at the same weight. The only factor to consider is discharge rates, but the sites I looked at werent terribly specific. With a lightweight setup and not too insane motor/gearing its a viable idea I reckons....