My midwife recommended that I start walking at least 30 minutes every day, or push to do 1.5 miles. She also encouraged me to do yoga to strengthen my muscles, posture and balance, without over doing it. I had already purchased a prenatal yoga card deck and DVD from amazon.com, so we will try those; plus our birthing center holds classes, for free, twice a week!
I started this a bit overweight, so I really want to ensure that any weight I gain is for baby, and not just because I feel that I can eat for two now.
Good Luck everyone :)