Dont expect too much too soon. But whatever you do, the next time you will do better. Promise.
I started going to the gym after being caught by one of those health check things in the library. It came up with raised cholesterol and raised blood pressure and put me onto the local Active for Life programme, which gives three months of free gym sessions and a mentor and eg free Weightwatchers. I'd been saying for ages that I needed to do 'something' but this booted me into it.
I didn't think i could run - crappy knees - but found I can run on the treadmill, the rubber surface makes it OK. Heard of the C25K programme - Couch Potato to 5km run. It means three sessions a week of 30 mins on that treadmill, or outdoors if you fancy. You-can-do-it Laura in my ears giving instructions. The first week I could just about run for a minute, then a minute of walking, then run etc. The next week, two minute run, 1 minute walk etc. I missed a few weeks with illness and builders in so it took about five months in all, but I can now, incredibly, run for half an hour non-stop. Well, by 'run' I mean a slow jog, but it does involve both feet being off the ground at once. I only do about 3k in that time so speed can follow later.
It helps that the council gym here is free for over-65's, a sound financial policy.
Seriously, if I can do it, it's worth anyone having a go if you can. I'm 66, three stone overweight but I haz TARGETS.
Still can't do the sit-stand thing though.