Since this sounds so much like my recent situation I gotta respond. A few weeks ago I was in pretty much the same boat as you -- always been interested in taking a martial art, never quite got around to doing it. I'm 27, and once upon a time I used to be quite active though lately the only workout I get is banging on a keyboard everyday.
I didn't really care about the self defense aspect, there's hardly anything I would ever fight about, and I do a pretty good job of never getting into a situation where I am forced to (I think the last fight I was in was over 13 years ago). Honestly I'm still trying to figure out exactly why I chose to take Aikido, at this point I guess it would be because what I have seen of it (technique wise) fascinates me, and as far as I can tell the philosophy behind it coincides with how I already view life and conflict.
I observed one class before joining and saw all good signs:
1. Everything was formal, but very relaxed at the same time.
2. Everyone got along and helped each other.
3. There was nothing wishy-washy about it (I find it pretty amusing the rep that aikido seems to get at various places on the web). Too many people out there who didn't get into enough fights as kids maybe
On a side note, I mentioned that I don't really give a hoot about the self-defense aspect (as in I feel no need to increase my ability to defend myself), but even after one class I have trouble comprehending how anyone could argue against Aikido being effective in "real life" situations.
Anyway, I've been going for a few weeks now and have absolutely no regrets. From everything I read prior to starting I figured progress would be slow, and it is (and I can see how it can be seen as much slower in Aikido than some other MA's). But every time I go I learn something new, and that's good enough for me.
Assuming that I will eventually figure out how to not roll on my head, and can quit smoking (lack of stamina is my biggest deterrent right now), I cannot imagine quitting.
Anyway, there's an opinion from a newbie for what it's worth.