If I wanted to run I'd enter a biathlon.

The closest I've seen to what you're describing is Zaandercross ("the Koksijde of the ‘Couve"). It has a long section that crosses a couple of beach volleyball courts, though I think more people end up walking than running. Running in loose sand is a different kind of running skill.



This year with the help of a little morning rain Erik Tonkin somehow managed to ride through the sand and gained a huge advantage over the competition. A couple of other guys on fat bikes were able to ride it too, but fat bikes are such an inherent disadvantage that I don't think it helped them at all over the whole course. It looked fun though.