It's definitely hard and it's kind of amazing in some ways that so many creators are able to do so
Like when I started my comic (which wasn't long running, per se, but did require a year to complete) I decided on the page a week format just cuz it seemed to be the hip thing to do. I was able to maintain that pace for... like... 4 months? And then my upload schedule started getting spottier and spottier until I wound up closer to a page every other week instead of every week by the end. And that's even with relatively short updates (single comic pages, so like... 4-8 panels on average is all?).
I think the only way to really make that work is to build an enormous buffer before starting, so enormous such that you could take a significant amount of time off and still maintain your schedule. I thought I was cute when starting with my like 1 month (4 pages) of buffer, and when I was fresh and enthusiastic I was even actively growing it, drawing like 2-3 pages a week. But I soon fell off that pace, ran out of buffer, and then the rest of the comic's run was stressful trying to meet deadlines on time.
The other option is just to do an alternative upload schedule. EIther upload less frequently at a pace that works for you, or maybe break it up in a logical way- like maybe you produce a whole chapter or season prior to upload, then set it to whatever upload frequency you want and begin work on the next one, have a small hiatus inbetween while you finish up the following chapter/season, and repeat. That way readers know to expect a break inbetween story portions, and if you need to take a slightly longer break that's fine... because you're already on break anyways. Just let people know it'll be an extra week or two or four longer than normal and whatever- folks who like your story will wait it out, and folks that don't won't be reading anyways, imo.