Thank you all for the helpful comments about yoga and spinning! They really were amazingly helpful.
I have survived one yoga class, which I would say humbled me, except that really, I couldn't have got any more humble about my yoga ability (except maybe tree pose; I can't get my off-the-ground leg above my knee, but I'm pretty good at standing still). My degree of inflexibility is astounding, though, and nothing else in my daily life requires me to stretch much of anything except maybe my mouse-clicking finger, so I'll keep going for a while.
I'll confess, though, that I don't think yoga is naturally my thing. I like activity that moves fast (even though I don't), to distract me from the fact that I'm exercising. There was not a lot to distract me from the fact that, oh, yeah, I'm supposed to be supporting the weight of my body on my left arm and left leg, and how does that work exactly? and now I just sit here? and we're going to hold this for how long? Is this done yet? However, I think it's good for me for precisely because it goes against my natural inclinations, so, again, will keep going.
There is one possible deal-breaker for this studio, though: the yoga studio is in the basement. The floor is nice wood and all, but it's not completely finished otherwise; the walls are painted and it's decorated, but there are pipes all over the place. And there are NO windows, not even the usual little casements hovering near the ceiling. And the lighting is kind of dreary (some floor lamps and then fluorescent ceiling panels, covered with a translucent material to minimize the fluorescent-ness).
Which, really, I wouldn't care - I'm not a diva about such things - but: I get claustrophic in spaces without windows. There were a couple of very fleeting moments near the end of class when I had the most ridiculous impulse to burst into tears - which is one of the ways this claustrophobia manifests. (Though admittedly the whole feeling-completely-incompetent-at-yoga thing may have played a part.)
It's a bummer, because this studio is literally three blocks from my apartment, they provide mats and blocks and so on for free, and I liked the instructor; not very hands on/directive, and not a lot of discussion of modifications, but generally clear, calm, and soothing without being overly spiritual about it (not to knock the spiritual elements of yoga for those to whom it's meaningful; just not my personal thing), and I liked the music she played. And I just took breaks when I needed them, and that was fine. And did I mention, three blocks away? Plus, pretty affordable, and a nice little local business.
So we'll see. I'll try this place again, definitely, but I might also check out the chain yoga place in town, which I'm pretty sure holds classes in a room with windows. The studio closest to me doesn't have any beginner classes at times that work for me (they're all 2 in the afternoon or the like), but another branch does, and is not horrifically far away, and at least in theory, if I continue this, I'll move out of beginner classes eventually, right? Right? So then I could go to more advanced classes closer to home.
And maybe if I'm very good, someday I'll be able to touch my toes without bending my knees.
(Next semi-major excitement: haircut and color tomorrow. Am trying to decide if I really want to cut off what has become legitimately long hair and go back to some variation on a bob, or if I will regret it as soon as it's done. Oh, dilemmas!)