Mileage rides are generally held in conjunction with Endurance Rides. They are normally 12 to 30 miles long. Riders are given a time frame within which they must complete. For a 25 mile ride, you might be given anywhere from 3 to 6 hours to finish. You may not finish too early or too late.
Mileage Rides are counted towards your mileage totals, and are eligible for the mileage program (if you are registered).