Assuming the trail marker is correct, then your computer calibration is too low. Yes, this affects both your mileage and your speed - both current and average. If you did the 20 mile trail in 60 minutes, it would 'only' register 19.2 mph average speed.

The best way to calibrate is to do a rollout. Start with your valve stem at the bottom of the wheel, above a crack. With the bike weighted normally, roll the tire 2-3 times, stopping with the valve at the bottom again. Measure the distance and divide by how many revolutions you did. If you measured in ft/inches, you'll have to convert to cm or mm, depending on which number your computer wants. (it'll be obvious - 210 vs 2100, for instance.) If you can't be bothered to do it right, then just guesstimate that you're about 5% low, and bump up the currently-entered number by that much.