I'm not sure what your goals are but I completely disagree.
Sprinting on the sand barefoot is one of the best things you can do especially for sprinting speed.
I've never run long distances on the sand but then again I'm not a long distance fan to begin with.
Running on sand is good because a. is an unstable surface and provides more resistance than sprinting on say concrete or grass (your feet have a harder time "grasping" the ground so it's more difficult b. is very refreshing being outside on the beach to me personally
Again, one of my goals is to be able to sprint like hell so I'd suggest short (10, 20, 40, maybe even 100 yard sprints). Sprinting short distances can even aid in hypertrophy/mass gains.
Yes, you have to consider the toll it takes on your body and can be just as taxing on your body as squatting/deadlifting if not even more but is equally beneficial.
Just my humble opinion.