I ran in the 1/2 marathon last weekend and now my ankle hurts. there was no obvious injury during the race?

Many things: During the race you may have simply turned your ankle. If it persists you need to see a doctor to rule out such things as ankle sprain, tendonitis, stress fracture.

Overuse: Could be overuse of the tendons around the ankle.See if rest helps..If not get an x-ray.

Most likely overuse: Follow the rice principle. Rest, ice, compression and elevation. Stop running until soreness is gone, ice 20 min 1-2 times a day, ace wrap as needed and elevate as much as possible. Overuse injuries are common in endurance events. Better training, shoe gear and supports can many times reduce this possiblility. Good luck. See your foot specialist if this persists. Dr l.