If your objective is to upgrade the bike, moving to 700c can make a big difference - lots of options for lightweight wheels, and lots of tire options. Upgrading components doesn't always equate to an "upgrade"... in fact, some more contemporary bits and pieces may wind up being heavier and in some ways less functional. If you go to 700c, you'll need longer reach brake calipers - Tektros worked fine for one of my conversions, and they look "right" with the 80's bike too. As has already been pointed out, none of this will make longer distances any easier. For distance, consider wider tires for comfort, bars and stem that dial in your fit, and a saddle that fits your sit bones. If you want miles, make sure your bike fits!

And post some photos when you get a chance. Folks here will come up with an unlimited number of suggestions for the build.