The cramps you've been experiencing are probably due to your shoes' lack of support. What you need is a pair of "Stability" or "Motion control" running shoes. These shoes will give you the necessary arch support, so that you can increase your millage.
I highly recommend you checking out local running shoes stores. They'll examine your pronation and recommend you a proper pair of running shoes. Also, go with the store that has a good return policy. My local store, Super Jock and Jill in Seattle, let me return the shoes after I've run in them for 2 weeks, about 20 miles. Their policy is to get me the right pair, and it doesn't matter how many pairs of shoes it's going to take.
Last but not least, the Propulse 2 might be good tennis shoes. But they would make horrible running shoes. No arch support. No forefoot cushioning. And the drastic heel-to-toe drop will only exacerbate the lack of forefoot cushioning.