The JFK 50 is across the river in Maryland. We don't cross the river.
I ran the North Face 50 mile race on the Virginia side of the Potomac. I was 49 years old at the time and wanted to complete a race with more miles than I had years.
Started at 0500 in the morning and just kept going until 1750 that afternoon, so just under 13 hours to complete the 50 miles. No speed demon, but I got the T-shirt and medal nonetheless.
Prior to this race, I had run a bunch of 5k's, a 20k, and one marathon. The 50 miler was in June. In May, I ran the Pittsburgh marathon and two half-marathons in preparation. One can do this without being some ultra-athlete as long as one has the perseverance to put one foot in front of the other, and repeat to the finish.
Something to look at is the nature of the course. A lot of marathons are road races, so asphalt all the way. The North Face race is an off-road, trail course with rocks, roots, stream crossings, etc. I am not sure what the JFK is.
It helps to break it down into 4 half marathons. I was able to schedule when I would be at one of the aid points. The long distance racers were allowed to meet "crew." My wife met me with a ham sammich, diet Coke, and clean socks. We sat at a picnic table, had lunch, and off I went.
The next day was no worse than the day after a regular marathon.