How do I know the size of my world, and its population

[Gumboot]

Not sure where you got your number but they seem similar to the one I have in my old dnd 3.5 edition book.

I created the table myself, from a wide range of different sources. I certainly wouldn't claim it as any sort of definitive source, it's just a good relatively realistic guide that I use to ensure travel times in my writing are consistent and realistic.

But I think the horse is faster even for longer distances, but since I have never been on one, I only rely on dnd and common sense.

They're really not. Horses can undeniably travel much faster than humans, yes, but only in brief bursts in the same way that people don't sprint or run, or even jog long distances, but walk.

There are different kind of horses too. Donkey ( not an horse but often used) is nearly as strong, much smaller and also slower, for example.

I think you'd be surprised how similar the walking speed of a variety of different animals is.

I mean the Spanish used heavy cavalry against the Moor that used light cavalry, much faster.

Sure, but over short distances, such as a battle. If you're covering a short distance, or you have way stations along the route where you can change mounts regularly, a horse is certainly a much, much faster option. But if you're going for long distance travel with the same horse, it won't cover it much faster than you will. And as Falconius excellently points out, there's significant logistical considerations with a horse.

There are also other mean of transportation such as ships but when I look at the table here : (scroll down) SRD:Movement - D&D Wiki
I get confused because the distances in my book are in km and in miles on internet and nothing matches but it's the same version.
Ship with sails can usually move all day long so that is why they have greater distance and some are faster than walking too, but noit too much. If someone could walk all day long, he could do 70km or more maybe. That is as fast as some sail boat it seems.

That's another great consideration. I didn't include vehicles because there's such a host of variables, particularly with wind-powered ships or river craft. I tend to calculate those sorts of journeys on a case by case basis.