One of the biggest concerns I hear from new mothers (who breastfeed) is the fear of losing their supply when/if they start running again.
It's a valid concern, because it can happen. Breastfeeding requires two things that running depletes your body of - calories and water. If you don't have enough calories to make milk, you don't make milk (or more realistically, you make less milk). Similarly, if you don't have enough "extra" water in your body, it can also cause you to produce less milk.
But there is an easy fix to both of these problems - make sure you're eating and drinking enough! Sounds too good to be true, right? Of course, for some women it is. Breastfeeding is funny like that. Each person's journey is so individual. But for most of us, that really is the answer. In fact, on days I run, I tend to produce more milk (at least according to what I pump) and I think it's because I'm more cognizant for making sure I'm eating and drinking enough.
Breastfeeding is not the time to diet. Cutting calories leads to supply issues. And breastfeeding itself burns a ton of calories. So adding the calorie burn of any exercise, including running, can compound the issue. Which is not to say you should sit down with an entire chocolate cake, you can still make smart food choices, but calorie restriction is out if you want to balance breastfeeding and exercise.
I'm already thirsty pretty much all day, a combination of the fact that breastfeeding increases your thirst and the fact that it's summer! But when I have a run planned, I add an additional 40 oz to what I normally drink (20 oz. before and 20 oz after).
My other big breastfeeding and running tips:
* Nurse the baby right before you head out! That way you won't be as uncomfortable.
* Nurse the baby when you get home! That way you won't be as uncomfortable. If you're gone long enough that whoever was watching the baby already fed the baby, then sit yourself down for a little pump session.
* Get yourself a "nursing friendly" sports bra. I love the Moving Comfort bras even when I'm not nursing, but they're especially fabulous when I am because of the velcro straps which means I don't need to try to play breast-tetris to get one out to nurse.
* Breast Pads! Even if you nurse right before you go, there's a chance the bouncing movement is going to cause leakage. And there's nothing more attractive than a woman running with two large wet spots on her chest.
What about races? For longer/bigger races where you need to show up hours before the start, and where you'll be running for a longer time that you usually go between feeds, I recommend getting a manual pump so you can pump to relieve some engorgement right before you run. You can also do this by hand, whatever you find easiest. I also highly recommend having your husband meet you at the finish line with the baby, so you can nurse as soon as you finish. This is what a lot of my friends and I do for half marathons while we're nursing. I'm sticking to shorter distances until December, when Thatbaby will be almost 6 months old, but with Thatkid, I did my first postpartum half marathon when he was 3 months old, so having him at the finish was a necessity!