I've learned to estimate such distances reasonably accurately, e.g. to the nearest mile, just by eyeballing them.
So, FingersR get you map and with your ruler look for things (small lakes, villages, VRPs etc.) that are 2nm from something else, and try to get your eye to see that distance as 2nm. Then do the same with 4nm and then with 3 nm and 1 nm. Repeat every once in a while to re-calibrate!
To add to Gertrude's suggestion, measure the distance covered by the width of your little finger (or a biro), so you can use your finger or the biro to judge small distances. (I have a 3nm little finger and biros are basically 2nm!)
Best wishes
tp