Thank you for your input, everyone. Gumboot, thank you especially. My world is certainly high fantasy, but that doesn't do much to dismiss my need to make things logical. I realize how much more planning I need to do the further I get into writing.
My main issue with population and civilization concerns was my own misreading of maps I've seen in other fantasy novels. They're not like Google Maps. They don't show you every little village - just the big or important ones. I think that was my biggest problem.
I found the page that Falconius linked shortly after I made this post. Gumboot's interpretation and personal insight definitely helped too.
As for the climate stuff, I didn't get a chance to research much about that, so the information Gumboot provided will be a great start to (re)designing my world.
Man, that might be too much research. But I'll probably make myself read it...
Hmm, that is definitely an interesting note, but I don't think it will matter too much. My civilization may include a bit of industrial elements to it so it shouldn't be too great of a concern. If anything I need to look into what the population did after that, but I think it's important to know the origins as well.
That is also very interesting and important to note. I'll see if I can find that thread.
I want to be done with the specifics already and just start writing again, but I know it would kill me if I didn't do it right. Thanks again for the help and pointing me in the right direction!
Edit: Gumboot, why do you say rivers don't split? I know they do, but is it just rarer than I realize?
Thankfully, I'm not as far behind as I thought and I don't have to change my plans too much. I have a better understanding than I expected, though I still have a long way to go. Again, thank you so much for your help!