I've got a 1985 model that I picked up about 8 years ago. I set it up as my all purpose bike and it has indeed been that - unpaved trail touring, year round commuting, errand running, hauling my kid around in a trailer all with minimal/cursory maintenance. It looks pretty beat now, but it has been a terrific performer. The 85 does have the canti's, though, and the longer chainstays (47?) of the 720.

The two components that have proven themselves beyond a doubt are the Schmidt dynohub and the Berthoud stainless fenders.

The Trek 720/620 are one of the all time great bikes imo. FWIW, my wife has a 1985 720 I set up for her.

Here is my 620 dropping my kid off at daycare: