Can you clarify what a sugar mash is? Just sugar and water, or did you mash some cereal grain? Sugar contains no micro-nutrients, so without a comprehensive yeast nutrient the fermentation might be nutrient-limited. Either way, 15 lb of sugar in 4.7 gal is *very* high gravity - about 37°P. Most washes for distillation are in the 5-15% ABV range, maybe 25°P at most.
Bread yeast isn't exactly noted for its alcohol tolerance, so even with everything else going for you, this wouldn't get to the ~27% ABV attenuation limit. Probably wouldn't even with a distiller's yeast.