My second week in I started doing yoga first thing every morning, about an hour, so I've been doing it about 2 weeks now. I do lots of standing poses, and I'm including arm and core work, so I'm building a lot of strength and endurance. Of course, I was in terrible shape to start, so practically anything would kick my butt at first.
But after two weeks I've seen a big difference. Instead of feeling like I'm about to pull something and hurt myself with every pose, I feel like I can at least approximate a rank beginner's version of the poses
And I am definitely already building endurance in the poses holding up my body weight. And while I'm not terribly flexible yet, I'm already better, and I'm not stiff and tight all the time.
I can turn clear around to look back while driving without sending my neck into a spasm :-) I can reach up more easily. I can squat and get back up more easily. I'm more comfortable when I go to bed. It really feels good, and I've only made a little beginner's progress.
I think progress will more gradual from here on out - just like with the pounds, the first few weeks seem to respond the fastest.
But I've been noticing small improvements (both in my practice and my daily life) every day since I started.