I have a few rides that cross the Nashua River Rail Trail on the road in several places... one that goes from Acton to Groton, through Lost Lake (hills??) and back and the other that goes up to Kimball's Fruit Farm in Hollis, NH and back here through Peperell, Tyngsborough, Dunstable, Littleton, etc.
I have been on it twice; once walking and the other last fall, when the last 8 miles of a group ride finished on the trail. Generally, I am not a trail rider, since they are filled with walkers, little kids, etc. that can be dangerous for cyclists. But, it was OK in the middle of the week, in October! I noticed in your pictures that those trails look a lot wider than the ones here.
I actually rode my Jamis Coda 50 miles on the Cape Cod Rail trail last June. Again, it was in the middle of the week, just before the real tourist season started.
It turns out that my son does know the woman who won the overall Longsjo 1st place, although not well. He said she was "very athletic" in high school and it would make sense.