This is a great place for all kinds of training, with nice views of old lava flows as well as good views of the San Francisco Peaks and Mt Elden.
This can be very hot in summer, so you may want to bring water, or stash some on your way up.
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.
From the parking lot, warm up on the flats and gentle rollers of Forest Road 545A
as you prepare for the consistent grade of the O'Leary Lookout Trail
. Once you actually start climbing, the road has a nice grade that you'll be able to fine tune your training. At 3.5 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 then 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.