That might be tight. The problem is that even with ADR's for 8:00, you probably won't get seated until 8:15, leaving you only 45 minutes to eat and walk down to the beach.
I'd try for a slightly earlier seating. Failing that, I'd get to Kona early and hope they can actually get you at a table at 8:00.
Which ever you do, be sure to tell your server that you need to be done by 8:50 so that they will keep the flow going.