The marathon rarely impacts park crowds. Resort availability, yes. But park crowds, no. The marathon (any race, really) does tend to play havoc with transportation as many portions of the races are along WDW roadways. BUT the races start very early (5am for the January marathon, I believe) and are finished by mid morning. If you're arriving noonish, the race will be over and any impact from it will be minimal for you.
There may be a slight uptick in crowds on Monday, but it shouldn't be particularly noticeable as most of the runners will leave Sunday night or Monday morning.
Steve