Ride as far right as is practicable - which may well mean that you will be in the middle of the road. You don't need a mirror - each vehicle operator is responsible for avoiding traffic in front of them, and paying attention to what is behind you is secondary. If there are cars parked along the side of the road, do not ride so close to the cars that you could be hit by a door which is swung open in your path, and if the lane is not wide enough to allow a passing car to pass you with adequate clearance within the lane, then you should make this clear by moving to the left, towards the center of the lane to make this clear to higher speed passing traffic.
If the road surface is not good on the edge of the road, that is another good reason to avoid that part of the road surface.