We lived in Kona, so I would say according to your requirements, you would not like the beaches there, although at your age and activity level, the Big Island would be a terrific fit, however if all you are looking for are sandy beaches why did you decide on Hawaii, if I may ask.
Hawaii is so much more than beaches. You should give it a try, the magic lies in the culture including the local people, the valleys, the untouched hiking adventures, the art, the snorkeling, the kayaking, not to mention always a wonderful view of the ocean, with perfect weather almost year round.
All of the islands are beautiful and have beaches. You may like Maui. On Oahu there are several beaches with trucked in sand that your toes will adore. You could try "best hawaiian beaches" on google for a rating
