Riding "just because" is fun, isn't it? I did that today - took the long route (around the reservoir) home from a meeting near the college. My bike is only a little newer than yours (1984) - love that steel

A couple of suggestions: new saddle, which you already know; maybe a new seatpost to get you farther back; try toe clips (I find they really do make a difference on a ride over 5 miles); bring snacks; and wear padded shorts, if you aren't already.