Coincidentally most Treadmills use a 80-260 VDC motor with a suitable HP rating and a PWM motor speed controller to allow the user to change the belt speed and keep a good constant speed and torque while running on it.
There are Commercial DC Motor/PWM controllers available or you can build the PWM circuit from scratch and buy all the components seperately but you will spend a lot of time and money either way. All the parts you need are on the treadmill.
Tear your own apart or get one on Ebay.
(shameless self-promotion below)
Motor/controller combos on Ebay
Safety and Disclaimers- You should have some knowledge of electricity and the dangers of household current and know your abilities/inabilities. Serious injury may occur to you or others from use/misuse of these motor set-up. If you are in doubt DO NOT ATTEMPT. IT CAN KILL YOU. Any Crazy Ideas found here REQUIRE your testing. Your appliction and use of any ideas here are all on you and you agree I cannot be held liable. You equipment should have On/Off safety switches, Fuse protection, ground wires on your machine as required and your power source should have ground fault interupters, circuit breakers,properly grounded sockets and cords and always unplug equipment before tinkering and any other safety practise I am forgetting to mention.