Do This
Run outdoors

Not This
Run on a treadmill

Propelling yourself on land is much more challenging than on an indoor treadmill, and it trains you better.

In a Utah State study, habitual outdoor runners completed a 5K run 4 minutes faster than treadmill runners. 

I'm very aware of the advantages and the disadvantages of running on a treadmill.  I personally find treadmills to be extremely boring even if I'm watching a television or listening to music.  I much prefer sprinting up hills in the great outdoors!  But what about bad weather days?  I much more prefer running in place on my "rebounder" or mini-trampoline than any treadmill.  The only time you'll ever see me on a treadmill would be when I have a routine stress ElectroCardioGram performed at my cardiologist's office.  By the way, when was the last time you had an ECG done?  If you haven't, I strongly suggest you do because it can show you what's going on in your cardiovascular system and if there's a problem... it can be helped and monitored.

BUT, if you absolutely insist upon the treadmill...

Do This
Walk 3 mph on a treadmill set at a 15% incline

Not This
Run 7 mph on a treadmill set at a 0% incline

Walking uphill on a treadmill is as challenging but easier on your knees than joint-pounding jogging, according to a study from A.T. Still University.

Whenever you use a treadmill, use at least a slight incline - when your treadmill is set at zero, the belt is helping you move, and that means you’re actually burning fewer calories than if you just went for a walk outside.