If the map is accurate at al for overland navigation, it needs to be somewhat consistent in two directions, and thus is often some kind of projection that is somewhat good for angles.
Now we need exactly one length, and everything else can be solved using Tigonometry: $$\sin \alpha = \frac a c\ ;\ \cos \alpha = \frac b c\ ;\ \tan \alpha = \frac a b$$ That's easy to solve for b and c, we just need the angle of one tip and everything starts to solve, and you will learn how long the distance between two more points is, and in the end you can map out the distance between any two points by trigonometry. The quality of your calculations is based on three factors though:
- your initial length measurement
- your ability to measure angles correctly
- the quality of the map when it comes to the angles being represented correctly
The initial length
Now, we need to know the initial lenth. How can you estimate that? In general, you can do so from the accompanying text.
For example, the rulebooks for the game Legend of the five Rings describe the distance between Kakita Castle and the capital as "a day's ride". That is not very precise, but can be a good starting point. Assuming that is a distance without a horse swap, we're faced with about 10 hours of riding (including a break in the middle), and a horse travels at roughly 3 miles per hour, so the distance is roughly 30 miles along roads. If the distance however is meant to be rapidly with horse swaps (like how postal service did!) a day worth in post riders is much further and for example the Pony Express traveled at an average of 12.5 miles per hour for 10 hours, so the two places would be about 120 miles apart, if that is a day's ride of the pony express.
Similarly, you might know how long it takes a military unit to march from one town to another. The typical, normal marching distance per day for an army that starts day with breaking camp and ends the day with setting up camp can be estimated quite differently. Estimates for the Roman Army swing widely, but typically are claimed between 30 and 40 kilometers, the typical average being about 35 km a day - or about 21 miles. That measurement can be taken generally as a good estimate to calculate distances between towns for any group that is not making extreme haste and is bound to the speed of a marching human.
If your predominant species is flying, measurements get much better by the way: they don't need to curve around terrain quite as much and thus the quality of the initial length is much better than with any ground based species. You just need to figure out how far they march in an average day. For example, the average Stork migrates in average 300 kilometers (~210 miles) per day during migratory season.