I'm writing a novel and I'm only in the beginning stages of creating the specifics and layout of the world (e.g. distance between cities, biomes, temperature, lakes/rivers, et cetera).

Assume a medieval setting (fantasy world).

Here are some things I've read:

Cities/towns should *generally* be spaced about half a day away from each other (so as to allow for travel to trade and return home before dark). So, assuming a wagon travels 3-4 miles per hour and the person leaves at 6 a.m. (give him two hours in town), the round-trip should take 10 hours. Making the towns roughly 35/2 (17.5) miles apart. Based on this, I would make other towns the full 35 miles apart so the trader/traveler could still easily make the journey during daylight. [This seems a bit odd to me. That is insanely close. It makes sense that people would live within these two ranges, but not that cities would be that close to each other. Historic maps don't support this either. Then again, maybe I'm just looking at larger cities.]

Truthfully, I hadn't really given much thought about biome placement before visiting this site. Even now I'm unsure how to handle them. Nature does some weird **** and for every rule there seems to be an exception. Using my own judgement, would I be pretty safe to place my biomes as I please, or is there a tool/rule I should know about?

Temperature is closely related to the biome concern. As with it, I would just assume I can trust my own judgement and common sense.

What about lakes and rivers? Again, I would just use my own judgement and common sense.

So, for now, I'm just going to keep using my own judgement and common sense on all of these points. This is a fantasy story after all, so some things are allowed to be inaccurate! If anyone has any advice on world design, please share it.