You should really consider an incline fast-paced walk on a treadmill. Or if you're looking for a much bigger challenge, go for an incline run.
If you're looking for something a little more challenging than a casual jog on a treadmill, it's time to crank up the incline. Some of the benefits of using an incline for the treadmill includes burning more calories and improving leg strength. A walk on an incline also decreases impact -- which can be beneficial to those who suffer from joint issues or injuries.
If you ever done an uphill climb, you know how out-of-breath it can leave you. The incline on the treadmill mimics that.