Well first you'll want to figure out what the section of the map you're building is going to be for. Depending on the use a map will be built very differently.
For instance a town map needs to allow for a lot of traffic, have places for players to hang out and talk, places for shops and stuff.
On the other hand a forest map needs to have places for monsters, a sort of 'main path' were all the people who are just passing through can travel on.

The other thing to remember is you want a lot of scenery. Don't just reuse the same tileset for everything.

Well there are two styles that you can use. The 'overworld map' style (like Final Fantasy VII) where you move around the main map, then when you enter a town/dungeon/whatever it zooms in. It's a fairly good style, although I don't like it that much.

Then there's the continuous style (like Secret Of Mana). Where you move from map to map.

The key difference is a continuous style allows for much more detail, and changes the feeling of travel. You really need to know your way around to travel properly in a continuous environment.

I've mapped quite a few games actually and my advice is to draw a basic layout either on graph paper or in paint. This is so you can use a pixel or block as a tile. Anyways you can always experiment easierly with the paper and or paint because it is alot less cluttered then the DM map and also you can see more. Anyways once you have drawn your basic map you can put it in. Though you may run into problems and may need to expand your roads due to player and npc traffic. like from a two tile road to a three tile road. don't forget to have roads winding all over and some back alleys.

As for distance between towns it really depends on the game. For example if you are living in a post apocalyptic atomic war towns would be spread out and very very basic. Anyways towns should be noticable no matter what. Like have an outer wall so players know this is a town.

Thats my advice i hoped it helped you.