My longest rides on a Brompton have been round about 60 miles, with no major discomfort. I'm contemplating a century, but I don't know when that will happen. YMMV though, as at 5'6", I find Brompton geometry fits me pretty well, and the saddle is pretty comfortable as well. I don't see how a long seatpost by itself would affect comfort, as opposed to where it is in relation to the pedals or handlebars. The Brompton is more comfortable than a friend's road bike (and the seat is agony after a couple of hours). It may not fit me exactly, but then he's just a bit bigger and he says it's a bit small. It's more efficient, so it's easier to go longer and faster, but my Brompton is easier on my body.