Good batteries and good battery management are the keys to extended run times with electric. Along with the above, regular maintenance to remove friction causing crap from the drive components and cleaning your motor will also help. Not being familiar with radio shack cars I don't know what kind of ESC they have, but if it's something you can change, going to a more efficient ESC will also have a big impact on run time.
Last of all, to quote a Dubro ad i heard: "Never buy an RC vehicle for which spare parts are not available."