I do fairly long rides on heavy bikes... but I worked up to it. Those first metrics on the Giant - at the end of the season - were pretty long & hard... but still fun. Big thing was I wasn't trying to keep up with anybody - our little group was tooling along at 11-12 mph. Rest stops every 10-11 miles.
For me, that was pretty "hard riding" - and there are generally large groups of folks doing exactly that same thing at any of the long club rides.
ON the other hand, lighter bikes *are* easier and faster to get going over long miles.
On the third hand, those heavier bikes are **great** for training - when I've been haulin' around the big bikes then those rides on the Trek are sweet.
I think, too, that Townies have a specific geometry so you can put your feet down... and yea, I would want to try it on short and medium and sort of long rides before I took it long just to k now I won't have a killer backache at the end (or chafing or whatever).