What? Really? 80% non-combat? Are you serious?
Wow. I don't think, back in the day, we ever had a session that wasn't 80% combat.
The AD&D campaign I ran back in high school starting at 3rd level with twinked-out Unearthed Arcana characters was combat-heavy, although I don't think it came close to 80% combat - I don't even think my current 4e campaign is 80% combat, and that's with fights taking vastly longer.
But the 1e, OD&D and BX games I've GM'd and played more recently eg on dragonsfoot have been about 20% combat at most, and that tallies with eg what I've read about the Dwimmermount campaign on Grognardia, or what I can glean from the 1e DMG about the designer's intent. The simple fact is that low level BTB D&D combat prior to UA was just so lethal that smart players learned to both actively avoid it and heavily stack the odds so that it was nasty, brutish and short - for the enemy.
I think some of the published tournament modules were significantly more combat heavy than this, usually with the expectation of high PC attrition, and usually being written for higher levels. But certainly when I ran non-tournament, 1st level, U1 Sinister Secret of Saltmarsh recently, we spent well under 20% of the time in combat.