IF you have the time, walking can be every bit as effective, like mentioned above. 500 calories is 500 calories, whether you walked or ran. Now, does walking get you the same residual results of building lean muscle and extending that calorie burn, I can't say.
As is recently evident, walking could be MORE beneficial in the long run, if you avoid injury and stay on track more consistently.
Dang, I'm not sure I DRIVE 70 miles a week