Before I got pregnant, I had this crazy goal of working out 200 days from conception to birth (so, if I gave birth at 40 weeks, then that's 200x in 38 weeks, or about 5 and 1/4 days per week). Then I revised it to 150 days (so that would be less than 4 days per week, on average).
I know it sounds insane, but I was actually hoping for consistency over intensity-- "working out" could be as little as one purposeful 20-minute walk. Unfortunately, during my first tri, I just felt so tired and yucky I was lucky to get in 1-2 walks a week, and nothing else. But I've actually done some sort of exercise 18 out of the last 20 days, so yay!
I need to make sure not to overdo it, but I think I'm okay in that respect-- it's mostly been walking, though mostly long walks (40-80 minutes), sometimes with DH. I added a little jogging a couple days ago and then was somewhat sore in my pubic area for a couple days, so I will go slowly adding that back in. (I had been a semi-runner for a few years-- ran my last marathon about this time last year-- until I injured a tendon last December. I have done minimal running since then, and barely since I've been pregnant.)
I also recently started a prenatal yoga class-- 1.5 hours per class, 1 time per week, 3 times so far. It's really more like 1+ hour of yoga and 20 minutes of meditation and it's very beginner-y, which is great, because I've never really taken yoga before. I figure it will help keep me flexible and injury free, and it's already helping with my weak lunge/squat muscles. I bought a beginner-y prenatal yoga DVD to supplement it, and plan to do it 2x/week at home with DH (who loves yoga), but have only done it once so far. I do like it, though!