Positives of running on sand:
1) if there is nowhere else to run, at least you can run on the beach
2) Beaches might have hot women in bikinis
Negatives of running on sand:
1) Sand flicks up your legs, really annoying
2) If you are wearing shoes, sand gets into your shoes and eventually causes blisters
3) Usually you end up running on a slight slant
4) loose sand it horrible to run on, no toe-off, puts strain in weird places on your legs without any added benefit
If you have to run on a beach, hopefully it is a beach with fairly "hard" sand and you can run down next to the water where it is slightly moist, causing it to be more firm and better for running. Also, hopefully there are lots of hot women in bikinis to look at as you run past. Longest I've done on a beach was 14. It wasn't too bad because it was a good beach. I wouldn't want to do that regularly though.