Lots of good advice from Destroy.
The write-it-down point is especially important, you may find stuff you create off the cuff while winging it is more inspired than what you laboured over pre game, and it shouldn't go to waste.
Re Wilderness "ELs" - I suggest that something in between 1e's "Orcs are always encountered in numbers of 30-300" and 3e's "Those hills are EL 1, those swamps next door are EL 12". IRL some wilderness areas are extremely dangerous, others less so but still challenging. I think 1e's approach where wildernesses were barely survivable before about 7th level was poor design and not very plausible, but neither would I expect 1st level PCs to conquer the north pole. I'd suggest that borderland or safer areas be mostly around EL 3-5, with a few more dangerous encounters - certainly in borderland there may be powerful fortresses and maybe the occasional dragon lair or similar. Other wilderness may be more like EL 6-8, with rare areas of extreme danger around EL 9-12. Those suggestions should be changed to fit your campaign demographics - eg if level distribution is 1-10 rather than 1-20 then maybe halve them (1-3, 3-4, 5-6).

Re Wilderness "ELs" - I suggest that something in between 1e's "Orcs are always encountered in numbers of 30-300" and 3e's "Those hills are EL 1, those swamps next door are EL 12". IRL some wilderness areas are extremely dangerous, others less so but still challenging. I think 1e's approach where wildernesses were barely survivable before about 7th level was poor design and not very plausible, but neither would I expect 1st level PCs to conquer the north pole. I'd suggest that borderland or safer areas be mostly around EL 3-5, with a few more dangerous encounters - certainly in borderland there may be powerful fortresses and maybe the occasional dragon lair or similar. Other wilderness may be more like EL 6-8, with rare areas of extreme danger around EL 9-12. Those suggestions should be changed to fit your campaign demographics - eg if level distribution is 1-10 rather than 1-20 then maybe halve them (1-3, 3-4, 5-6).