I was thinking this very thing while watching the coverage Sunday morning. I think what it comes down to is whenever a marathon is on tv the US networks try to cover it the same way rather than tailoring the coverage to who is likely to be watching. Look at Sunday's race (and the women's race a week ago) and compare it to the Boston Marathon on NBCSports and the NYC Marathon on NBC...
--Boston is live on a specialty cable network, so the likelihood that random viewers are going to stumble on the broadcast is not high.
--NYC is same day tape delay on a major network in the heart of football season. You might get a sports fan as they flip through the channels, or someone who isn't that into football but likes sports.
--The Olympics were live but at 6am. For different reasons than Boston the likelihood of random viewers watching is pretty low.
I don't believe everyone watching marathons on tv is a major running fan, but certainly the vast majority of the viewers are more knowledgeable than the average viewer of sports on tv would be. A few features are fine, I thought the profile on Meb Sunday was well done. For NYC and Boston due to the nature of the programming I think you can work in some local features and do it well. With NYC it probably has more potential because of the time it airs as well as being on a major network. But that coverage is severely edited for time. If you're going to air a 2 1/2 hour sporting event and smash it down into 2 hours of air time (~1:30 of actual coverage) showing a bunch of longer features doesn't work. Take a cue from MLB coverage, if they're going to do a feature (for example an "in the clubhouse" interview) during an inning they cut to it while still showing the action.
One thing that I really thought was lacking during the coverage of the Olympic marathons was any background on why the runners up front were contenders. Those of us on this board might have a decent idea of who the Kenyans were, as well as the Americans, dos Santos, Rothlin, a few other guys. But even those knowledgeable about the contenders can use thumbnail profiles. It should be easy to pop up a box that tells us what Mutai or Meb or Kiprotich (to name 3 at random) have done in this season and the past. Especially for the Olympics, where you have 3 per country, that kind of graphic should be a no-brainer. I'm thinking something like UFC does with their Tail Of The tape. It's not like this info is hard to come by, a production assistant could whip this kind of thing up quickly.
The other thing I thought was really lacking was any kind of substantive coverage of how far ahead/behind runners were. That info was readily available on the IAAF site. Given how much was made of the Kenyan/Ethiopian rivalry in all the distance running events I thought it was a heck of a story that the Ethiopians faded over the 2nd half and none finished. No mention during the coverage. On the other hand, Meb going from 19th at 20k to 10th at 30k to 6th at 40k to 4th at the finish was a great story that got no words. I can only assume that the announcers never bothered to look at that info.
If your viewing audience can be expected to be educated about a subject, whether it be marathons, Nascar, cooking, whatever, you need to entertain them in a different manner than if you're broadcasting to a broader audience.