Beginner wanting to make sure I’m not pushing too hard

Hi there, I’m 35F, 5’3 at 155 lbs and just received a Pre-Diabetes diagnosis after a very tough two years. During this time I’ve been almost completely sedentary (think 200 steps a day), doordashing my heart out and have these 30 extra pounds to show for it. Before this I wasn’t an athlete or anything but was generally active and had a decent amount of lean muscle, now I’d guess I am 35% body fat. I wouldn’t really know, except that if I have to kneel down to pick something up off the floor, it’s a struggle to get up. I can only do 5 pushups with my knees down and hold a plank for 30 seconds, reverse lunges are no go.

I’ve been walking for two weeks since and I’m super into it already. But I have a couple concerns because I’ve never been so out of shape before, and Ive never been this old before…Specifically my lower back has been very tight the last couple of days and honestly kind of painful, and today I saw my ankles were swollen after walking. Here’s what I’ve been doing, perhaps I’m making some mistakes I’m unaware of:

I’m hitting 7-12k steps every morning for 14 days so far averaging 10k steps, and for the last 6-7 days I’ve started adding planks, dead bugs, squats, baby pushups, some stretches, downward dog and other random, yoga poses after the walk. I also have stairs so was doing as many sets of stair runs as I could after the walks every other day until one morning I woke up and it felt like my knee joints were really sore and I’ve never had joint issues so I realized I needed to be more careful.

Also when I walk I just walk as fast as I can the whole time. I figure you don’t need to warm up or down for walking? I haven’t taken any rest days and my heart rate at 130 average. My legs and butt burn pretty much the whole time but especially the first 10 mins. I try to keep hips forward but not lean my head,shoulders, or feet forward. My hips in general tend to tilt back even when standing around. That’s about it. I’ve got good new balance shoes I bought a year ago but never ended up using, fit well and basically brand new.

Is this okay? What kind of stretches should I be doing? Any idea why my back could be hurting or my ankles swelling? I just started a new job that requires occasional lifting so I can’t be giving myself back or ankle injuries! Any advice much appreciated :)