Hi -
I run long-distance too and did before, during and after both pregnancies - kids are now 7yrs and almost 4yrs. I had the same thing - lots of milk and BF until my oldest was almost 2. The second time, I didn't have as much milk, and she seemed to pass me up (I needed to supplement with #2 and never did with #1). But, I still BF her until she was 15 months old. I don't attribute the lack of milk to running though. I think it was just a combination of having more going on and not making myself drink more water, chasing a 3 year old, etc...
Just remember to hydrate more than you would normally. I talked to a friend of mine in a running club who said that while she was nursing she only ran on a treadmill so that she didn't overly tax her body by sweating outside in the Texas heat.
Hope this helps!
C.