L.Z.
I ran early on in my pregnancies, up to about 4 months, then I just couldn't deal with the constant need to pee and the heavy feeling, so I switched to walking. I ended up having to buy new sneakers midway through each pregnancy (you have done well just noticing the bigger feet feeling now!!). I'd get a new pair and try to get them cheaper, as much as it stinks. Your feet may even stay "bigger" for a little while postpartum, so you may use them longer than you think, like the first post said.
One thing - I'm sure you are exercising smartly based on your fitness level and background, current pregnancy, etc. Don't let others make you feel like you are harming your baby by exercising, you are doing one of the best things you can for both the baby and yourself, and not just from a weight gain perspective! People have a mindset that once you are pregnant, even right after conception, you shouldn't exercise, and that is not accurate if you are healthy and have been working out to begin with. I hate that attitude... just a vent here! I hope you keep it up and have a fabulous recovery from the birth, because it helps with that too!!! Hang in there!