If you can do 50 miles you can do 100 - just time on the saddle. How much have you ridden in the last few weeks? It's a really flat ride for the most part.
In terms of the folder I think Avatar will be on one - he likes that position more than his road bike for the longer rides. I won't, it's a bit uncomfortable on longer rides for me (could be a bit taller for my legs). Really depends on which bike you'd want to spend 6-7 hours on
There's another side to the story... because of my body's proportions (5'7.5", long body) the Dahon Speed P8 offers me an alternate position, which allows me to go faster: I can lean over with my elbows relaxing on the handlebars for a racing position. The Downtube has shorter handlebars, so I don't know how the stability feels if I were to try it on that.