How are you running? Are you setting it to an incline or varying your speed? I find a 40 minute run takes me 4.5 miles with about 430 calories burns.
You burn calories throughout the day anyway, so obviously you don't want to try burning over 2000 calories on a treadmill.
I would try to vary the kind of cardiovascular exercise I was doing, because running all the time would make me bored, so I switch to the cross trainer or rowing machine sometimes.
Also, doing weight training is highly beneficial when it comes to burning calories, because when you, for example, run, you burn calories when you're running, whereas when you're weight training, you also continute to burn calories for a significant amount of time afterwords, whilst toning up without worrying about bulking up.
Hope this helps.