Yes. Of course, treadmills are an even incline, whereas the slope of hills can vary, so the slope may include sections of 10% and sections of 14% for an average of 12%.
15% is a considerable incline, and about 2.5 times as much effort as walking on the level.
But 2.5 mph is a fairly moderate walking pace. If it is feeling easy, perhaps you can up the pace?
Oh, and I assume you are not holding on to the hand rails on the treadmill? (this means your body weight goes down through your hands rather than your legs, and makes things much easier, and thus the training a lot less effective).