Cheat: Unlimited Miles
1. Go to Time Trial mode which is (Singleplayer).
2. Choose a player with a Motorcycle (they can recover faster).
3. Choose Tokyo-To Shibuya Downtown level (this level has the least amount of obstacles and drop offs).
4. Use tape or a rubber band to hold down the gas (L Trigger / L2) and just let your character drive though the course over and over again.
It takes about 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