DorkbotPDX people doing strange things with electricity

I am fairly new to robotics/autonomous vehicles. I have a quick question; maybe someone can answer for me. I have a little Hot Wheels RC car that I had gutted all the rc components off of; I am wanting to attach my dorkboard and control the speed and direction of the motors. I have a circuit design to produce a motor controller (H Bridge)( http://itp.nyu.edu/physcomp/Labs/DCMotorControl ), but I would like to control the speed of the motors (14mph is too fast). How will I combine motor and speed control in one circuit and then use the dorkboard to control them?