People have ridden LEL on one.
I think this is going to have to be a Rule According To Kim™: Just because someone's completed $epic_audax on one doesn't actually mean it's appropriate for riding long distances.
I've ridden mine 80 miles, several times. It depends on your tolerance for a slightly sluggish ride, narrow grips (on a Brompton) and a bit of a pounding on rough roads due to the small wheels. The flexibility when pedalling is something you get used to after a few miles - Bromptons have a fore-and-aft squidginess unlike a normal bike - and they can go up and down hills well enough.
I think the most I've done on my Brompton is about 55 miles, but some of that counts double due to pavé (I was swearing about how next time I was bringing the mountain bike for much of that). I certainly wouldn't consider 16 miles too far, though 5 miles is more typical for what I regularly use it for.
The narrow grips on a Brompton problem is surely only something that affects people with big hands and non-S bars? (Though mine has the grip shifter problem of knobbling your thumb if you spend too much time on the flats.) Regardless, if you're going to be doing non-trivial distances, bar-ends are going to be a worthwhile upgrade.
Agreed about the rest, though my experience is that the sluggishness make them fairly rubbish for descending too. Climbing is the one thing where the Brompton doesn't feel slow.
Nevertheless, larger wheels would certainly make the whole thing a fair bit more pleasant. 20" Dahons, for example, are much nicer bikes to ride than Bromptons. It's working out how much you value the fold.