If you want to try a cheap motor controller that gives you variable speed, current limiting, and low voltage protection (to save your batteries), check out these scooter parts.
http://tncscooters.com/partsdb.php?type=ES looks like a $12 one might work for you.
The electric scooter I recently aquired uses many of the parts they sell and the variable speed drive is pretty cool and supposedly 95% efficient.