It isn’t really summer until the Great Duck Race begins, and for last week’s running, Lorilee Fish was selling bags of rubber ducks hand over fist for the final time. Fish, Mansfield’s director of Park and Recreation, started the race 21 years ago as a way of raising money for summer recreation programs. She is now retiring, passing on the ducks — more than 3,500 of them — and what she hopes will be tons of fun to the newly named director, Sherri Gurnon. “It just exploded,” Fish said of the race that sends thousands of ducks, each marked with a number, over the dam at Fulton’s Pond. About 1,000 people turned out for the tradition last year. Ducks cost $1 apiece, and the first 100 or so to cross the finish line at Kingman Pond win prizes donated by local merchants valued at up to $25.