Don't worry about it. As some previous posts say there are different types of fitness. I can easily run a HM in well under two hours but put me in a pool and I struggle to do two lengths without getting tired!
I read somewhere (sorry, I can't remember the source) that it can take up to four weeks to recover fully from a Marathon; you just did "two Marathons" - so your body is going to be really knackered. As a poster above said, your immune system is compromised during that period post Marathon. I am aware that you can do recovery runs a few days after races. The three mile run that you did - if it was fast then it may have been too fast for your recovering body as I understand recovery runs should be nice, gentle jogs.
I have read this in the actual magazine I got last week (the full article). Some of the statements/quotes need to be subject to far greater peer review and scrutiny - especially the stuff about saturated fats.