N.,
Great job on the marathon - doing Chicago? I'll be training for my fourth after my little one's born in late Sept (plan to do Chicago 2010). Anyway, I started running again shortly after my son, now 2, was born. My husband decided to take up the sport with me at the time (he has since quit), and he was pushing the running stroller. When he quit running, I had to take over that load, and boy was it hard at first! Pushing all that extra weight really makes it more difficult, but it will get to the point where pushing the stroller is nothing, and running without is WONDERFUL! Anyway, with the knees, it's probably a result of your loosened joints, as another mom said. I would do some research on this, as you might want to purchase some knee braces just to help keep everything in alignment and make sure it doesn't get worse. As far as milk supply, you HAVE to make sure you're getting enough water and calories per day to keep it up. People might notice a decrease because they are not increasing their caloric intake. Keep in mind that when you run 1 mile in 10 minutes, you're burning about 100 calories. If you're running faster than that, then you're burning more. If slower, slightly less. When you run for 30 minutes, consider it 300 calories. You need to eat this amount in order to keep your milk supply up. Even if you're trying to cut calories in order to lose some of the extra baby weight, you have to be very careful and slow with this process. Make sure you follow your run with a healthy snack (in addition to your daily meals) consisting of some carb and some protein, and drink drink drink the water! If you're noticing a dip in milk supply, eat a bigger snack, even if you're not hungry. If you're eating enough calories and drinking enough water, you should find no significant decrease in milk supply. Since your little one is starting solids soon (or already has), then you will most likely have a natural decrease, since he won't be getting all of his nutrition from you.
Well, I must say that I am SO jealous of your ability to run. I'm 7 1/2 months pregnant now and WAY too big for jogging. I miss it so much! I got a great deal on a double jogger off Craigslist and I can't wait to use it next summer!!! Best of luck to you in your marathon.
Amy K