Extreme obesity

Well, it's mostly just getting heavier. Everything takes more effort, nothing will fit, and what does won't fit right. Depending on diet and age, digestive problems are common, and so is joint pain. Sweat everywhere, almost all year.
Mostly it's small things. Stairs become a chore, and so do small cars low to the ground.

I mean, I'm used to stairs. I walk a fair bit daily. It's just harder to do, requires more energy (and more calories.) Some things require flexibility that is just no longer possible with a more voluminous body.
Another thing, at least in my case: I never feel "full" or even "sated." It's like my brain says, "you have more room, may as well eat up."

Depending on where most of your 400+ goes, will be a matter of adjusting. If you gain slowly for your muscles to adapt, your mobility (while a challenge) will be easier than rapid gains (especially lower back pain if it is mainly belly).

Be wary of Some, smol-fats (especially new ones seeking to be an ally/friend) as some physical trainers/others advise them to seek you out to push food on (and help them adjust from the drinking/stress eating/other habits).

Friends/coworkers may invite you (more often) to the house with their wives etc as you're "not competition". That might change when furniture breaks or the adjustable seat doesn't go down (like before).

Best of Luck out there, as it's not 4 everybody (especially longterm).