When I do WorldBuilding, I have to stop and not get lost in the detail.
For Legend, look at what you absolutely need for Adventure Creation, Backgrounds, Professions and maybe Cults/Societies. OK, you need Backgrounds, so decide which backgrounds are available to the PCs, that will help shape which cultures are around and will also shape the Professions available. If you want cults, then go into the barest detail possible, perhaps a couple of lines showing what the deity is famous for or who worships the deity and the spells granted. No need for a full cult writeup, that will follow if the players are more interested.
Once you have an idea of the cultures, you can say whether they are close by or far away. For far away, just put a sentence for each and then ignore them until the players ask about them. If they are close by, then make a list of where the PCs are and what is nearby. Decide on layers, so "Here" is what is actually at the place, "Nearby" is what is around the place, "Close" is a couple of days travel, "Far" is anything more than a week's travel. Have a sentence for anything that is "Far", a couple of sentences for anything that is "Close" and more, if you want for anything that is Nearby or Here. Describe through Adventuring, not by Narration. So, don't give a 20 minute lecture on all the places in the kingdom, players will get bored and won't remember anything. Instead, say "Farmer Giles has had some sheep stolen from top Field", so the Adventurers get to go to Top Field as that is Here, "One of your friends has been attacked in Pigbutt Inn" means you know there is a place called Pigbutt Inn, so you can go there and find out where it is.
Build up a picture slowly as the campaign goes on. Fill things in as and when you need them. Don't write everything up front as it is a lot of work and you would probably only use 20% of what you have written. As you write more and more, save it somewhere so you can refer to it later. If you are technologically inclined, set up a Wiki or something for the players to refer to, if they want.