score:5
Leave yourself extra time to get to the airport. Other than that, unless one point of your journey is on Staten Island, or literally along the course route, you probably won't even notice the Marathon.
NYC is a huge city. The Marathon is a big event - and as you've noticed, the course sprawls all over town - but because of the somewhat mobile nature of the marathon crowds, it tends to not be as big of a deal as some make it out to be.
In terms of tourist attractions, anything south of 59th street in Manhattan is going to be 100% unaffected by the Marathon. Which is a chunk of the city that contains, if not most of the tourist sites in the city overall, certainly the largest single portion of the cities sites of tourist interest in an area of it's geographic size. Really, the Marathon doesn't really hit much of tourist interest at all. Other than it's jaunt through Williamsburg along Bedford Ave. (if you're interested in shopping or brunching with the hipster-set), and the run down the UES/East side of Central Park (with the various museums along that path), the Marathon largely stays out of NYC's tourist districts.
As far as getting to the airport - the loss of the Queensboro Bridge hurts. Especially if your destination airport is LGA and you're headed there from Midtown Manhattan or the UES - it's a favorite of cabbies for the lack of a toll, and of course, any traffic it would normally carry is probably being redirected to the Midtown Tunnel. But it's far from the end of the world - give yourself a little bit of additional time (I'd say an extra hour to be safe, but you can cut that down the later in the day you need to go) and you shouldn't have anything to worry about.