Surely the controller has at least 50v caps in it. So assuming it's not a high amp controller, a pingbattery 36v 15 ah would be a great choice. You'd have 20 mile range or more at15 mph, and for sure 15 miles full throttle.
If the controller is marked 15 amps or lower, then you could economize and get the 10 ah, if that would be enough range for you.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.