I think it really depends. Working out (C25k and 30DS) never messed with my supply, but I remember hearing ladies talking about exercise causing their supply to tank in the first year.
I had no issues with working out extremely hard and nursing. I did, however, have to drink a TON of water, and I don't think I would sweat/cool as efficiently. I trained for several races while still nursing my son, starting from the time he was 6 weeks old.
Re: Working out & Breastfeeding
Just out of curiosity, was this your first 1/2 or are you a seasoned runner?
I'm planning to go back to training a soon as I'm cleared to workout again. However, I want to be able to breastfeed.