Third trimester - walking still OK? (I walk a lot.)?

It's totally fine, unless your OB told you not to. I worked on my feet, 8 hours a day 5 days a week up until the day before I went into labor. As long as you are taking it easy, getting rest if and when you need it and making sure you stay hydrated, it's perfectly fine. I think that me walking so much helped a lot with my labor, hopefully you will be as lucky as I was. My labor was only 3 1/2 hours. I gained a total of 24lbs my entire pregnancy. Sounds like you are on the same track I was. If you feel comfortable doing it, then I would continue to do it. Let your doctor know you are walking a lot just to make sure with your pregnancy history, that it's fine. There shouldn't be an issue though. Good luck!