On multi-use trails I personally believe if you're going faster than someone else it's your responsibility to figure out the passing maneuver and it's your fault if there's any collision. If I see walkers with ipods I slow to nearly their speed and pass very casually. There's no need to be in a hurry to blow past people. If you are in a hurry, take the roads. Think of it as great exercise to accelerate back to your preferred speed each time.