You won't break anything. The motors will naturally get hot, but you should still be able to touch them. If not, then it's running on too many amps. You can hear the motor's skipping steps if the amps are too low.
You can increase each of the 3 knobs (potentiometers) to full. On your CNC software (Mach3?) you can set the acceleration and feed rate. This will increase or decrease the amps to compensate -- so it can be controlled by software. Again -- just monitor the heat.
If you are going to be running it for a few hours, it wouldn't be a bad thing to put a small fan to the enclosure. Just make sure things don't get too hot.