Ceterus Parabis (all other things remaining equal), upping the voltage in your current set-up will result in the power required increasing as well. This means that the overall run-time will be the same or less (perhaps a small amount of extra current too).
In this case, you need to increase the capacity to gain additional run time.
If you change to a higher voltage of the same capacity, then change the motor to one better suited to the voltage, and the gearing to gain the same top speed as before, then the runtime will increase, due to the same power requirement, but the pack is able to provide more power as a whole. This set-up would also be more efficient, and thus further run-time is gleaned.
In this case, increasing the voltage, decreasing the motor KV and keeping the top speed the same will increase run time.
__________________
|