In the 25 years that she worked overseeing Todd Pletcher’s racing stable, Maggie Sweet only had to concern herself about horses from the time they were two until around five. In the one year that she has been executive director of the Thoroughbred Retirement Foundation, the nation’s oldest Thoroughbred retirement facility, she has learned that inspiring the industry to spend the money to care for them for the next 25 years is a much harder part of the bargain.