new in the world of electrics, just a few e-revo questions

ok guys i know there are alot of e-revo threads out there but i have not found one that will answer this question. I have looked at some of setups out there for a e-revo, and they are using 8000mAH 4s lipos and getting 15-20 min run times. and other setups are using the same batteries and getting 45-to and hour run times. How is this possible?? And how do i build one that will run that long?? cuz if im gonna spend that much on batteries i want to run for awhile on them....But i also want to go at least 40-45mph... is that possible???