That is crazy. A shop built wheel is shot in 50 miles? Take it back to the shop, get a refund. Find a good wheelbuilder.
I'm only 230 pounds. My front road wheel has 16 spokes and my rear has 21 spokes. I train on these. I have different set with probably 100,000 miles on them with 20 spokes on the front and 28 spokes on the rear and not only have never broken a spoke but have only trued the front once. Both are carbon rims.