It depends. A modern triathlon frame is comfortable whilst in aero if you are fit correctly, but for the majority of people a Lo-pro position does not lend itself to a long and easy day in the saddle.

The geometry is built around a standard seat tube angle (73*) so when you get into the aerobars you stretch out and flatten your torso making your lower back muscles support your trunk. An extreme example is Obree in the Superman position, just to give you an idea of what I mean:



The stretched out position is further exacerbated by the extremely low head tube and base bar height of a Lo-Pro. These bikes are designed for one thing only, and that is for going extremely fast in an aero tuck at the expense of comfort.


Compare this to a ´steeper´ triathlon frame, built around a 78* ST angle, which enables the rider to keep the upper arms vertical in aero and support the trunk better, as Adriano Malori demonstrates without the bike:



See that 90* angle from torso to upper arm, and from upper arm to forearm? That is what is currently considered the optimum fit for a long distance aero position.

and here´s a nice picture of Sindballe, reckoned to be the gold standard as far as tri positioning is concerned:



He can hold that position all day and run a marathon off it.

I think a weekend ride on flat roads..... would be OK, there would be better tools for the job though, plus you would be constantly having to wait for your friends. Hope this helps!