Running that motor on an L293 you will surely burn out the L293, as it cannot handle the stall current. So, first, you need an H-bridge with a much higher current rating. Second, to control that motor without cooking it, you MUST have a means of monitoring and limiting current. That means using a motor driver that gives you a current feedback signal. Those motors will go up in smoke after a VERY short time at max current. Ask me how I know...
The only way you can reasonably achieve anything close to constant RPM is to use a PID controller to regulate the speed. Even then there will be a significant minimum voltage required to achieve ANY rotation, much less your target speed. The motor will not even begin to turn until it sees something on the order of 3-5V, as both the motor and gearbox have considerable internal friction.
Regards,
Ray L.