A lipo pack of around 3800 mah will have the same capacity as a NiMH at 3800. Having said that, a 2s pack at 3800 will probably last SLIGHTLY longer due to the lower weight.
Get a 6000 or 8000 mah pack. Getting a higher capacity pack will allow the average voltage during that run to be higher and still weigh less. Plus, if you get 8000 mah, you can run 2x longer.