I've run JFK 50 miler. First two miles are on a two lane road....very easy to socially distance.
The next 14 miles are on mostly single track (with a few spots where it opens up.) Definitely could be a log jam in parts. But you are also out in nature. But there are sections where staying 6 feet apart is impossible if you want to pass someone. However, the camaraderie is such that they could ask people to alert someone if they want to pass and both parties put on a mask while the pass occurs.
The last 34+ miles is on a wide open trail and road, so social distancing is easy. The finish is not chaotic at all with plenty of space. However, I did have to ride a bus from the finish back to the start.
My feelings are the race could be done safely and I hope it still goes off. I understand the concerns about travel and I'm not underestimating those risks. I'm a regular mask wearer and definitely not an anti-mask guy. I think masks work and are critical and should be required, however, I also feel like we need to find safe activities to keep ourselves sane. The Boston Marathon is not one of those activities. JFK is something that is very low risk IMO.