The very same thing occurred to me last year - catching the running bug that is. I too started off with a couple of 10k races, then decided the ultimate challenge would be a marathon, which I trained for and completed. So, did I weight-train whilst running?
At first, yes, as I built up the milages. But before long running 5-8 miles 3 times a week then up to 20 miles on a Sunday becomes far too taxing whilst still lifting. Not to mention the change in diet - to run you need to stock up on those carbs in serious quantities, and this just doesn't work when only lifting, when a great deal of your calories have to come from proteins.
In the end I gave up the lifting until after the race, then took a month off ALL exercise - now that is serious SD time.

I'm back into the HST now, and do short runs (up to 3miles)either after my workout or on in-between days. I lost about 20lb through the intensive marathon training (5 months), and went down to single-figure bf% for the first time ever. Sure, I lost a good deal of muscle mass, but now I am building on a much fitter, stronger and aesthetically pleasing frame.
Running IS a bug, and not one I can ignore I am afraid. Dependant on your goals it CAN complement HST (or any weight training). You just need to define the goals clearly...
