Your decrease in runtime could be due to your wearing out the battery. Standard car batteries are *not* designed for deep discharging - they're designed to pony up the initial high current draw in order to start. If you're going to be running it off the battery alot, I recommend you getting a deep cycle battery.
Getting it to run longer really comes down to a) decrease system draw b) increase battery capacity. If you have the room, the easiest (and cheapest - there is a thread that mentioned getting used wheelchair batts free) way to significant increase is to y use multiple batteries in parallel.