O'Leary Peak is a great hill to get some good elevation gain as you train and take in the views of Flagstaff. The "trail" is a dirt road that leads to a fire tower on top.
Once you start climbing, the trail has a nice grade that you'll be able to fine tune your training. After a couple of miles from the start you'll come to a gate, this is a nice place for a shade break or a turn around point if you are in a rush to get back to your car.
After the gate, the gradient increases and stays steeper than it was at the start until you get to the top at the fire tower. The fire tower is still in use so you won't be able to go up the tower.
Take in the views at the top, and then turn back and follow the same route on the way back to your car.