My doctor said that as long as I am still able to hold a conversation while working out (ie- I'm not getting out of breath), I stay hydrated, and stop if I feel any pain/discomfort, I would be fine. He said I could pretty much do what I did pre-pregnancy, with the exception of some weight exercises and anything that involved me being on my back for a prolonged period of time.
Walking is fairly low-impact, and I was actually encouraged to keep doing so while pregnant (at the gym/outside I will walk up to 5-8 kms at a time). I would definitely say speak with your doctor- but generally your body will tell you if you are overdoing it. Gaining too much weight is a real concern to me (my wedding is less than a year after the baby is born, and I was previously heavy and lost a lot of weight)- and there has only been one time where my body went "woah, slow down". Don't go running any marathons, but some low impact cardio is likely fine
