Treadmills can be used for interval training and are especially good for hills.
Hard: __mph at __incline for __time
Easy: __mph at __incline for __time
With each successive interval, it will become more difficult as you tire.
The treadmill can be ruthless in that regard. It says, keep up, or you're going off the back end. You can't get a tougher coach than that. It's repeatable so you can track your progress.
Hill training on a treadmill does not require the big motor since steeper inclines require less power. There are usually a lot of used treadmills for sale on Craigslist.