Go to Time Trial mode and select a player with a Motorcycle (they can recover faster). Play on the Tokyo-To Shibuya Downtown level (this level has the least amount of obstacles and drop offs). Now, use tape or a rubber band to hold down the gas button and simply let your character drive though the course over and over again. It takes aroundt a half hour to earn 15,000 Miles. Your controller may shut off every 15 minutes, this is normal -- you'll have have to turn it back on to allow this to continue.