See the FAQ about long distance on a folder. Any folder will do long distance as long as you are comfortable and with suitable gearing.
I have a Tern Verge S11i which I bought almost 4 years ago. I rode it on a charity ride for the Leukemia and Lymphoma Society in 2013: a "Century". It was quite comfortable. The only modification I made prior to the ride was to install a pair of clipless pedals so I could wear my regular cycling shoes.
On the frame reliability issue... Last year my frame was recalled and replaced. It wasn't cracked, and I was reluctant to submit to the inconvenience of the replacement process. Tern twisted my arm a bit and I complied.
The new frame has internal bracing forward of the frame joint which appears designed to prevent the cracking problem.
The other improvement in the frame is the frame hing pivot. It is a single rod instead of the two bolts inserted from opposing ends of the pivot. This should eliminate the hinge pin failures that have plagued early Tern frames.
I'd buy another Tern today if I were in the market. The Verge S8i would be a strong favorite... I only wish it had the Alfine 11-speed hub. I suspect the 8-speed hub is used to keep the price down. It's a good hub, but I like the 11-speed which gets its oil changed every 600 miles or so.
Steve
Attached: 1) view of the under-side of the frame showing the weld for the internal gusset plate, 2) view through the vent hole in the front frame half showing the gusset plate, 3) the new frame installed.