"American football" (what the world calls it, to differentiate it from "real football") is a small stage compared to Olympic track and field. Goodwin is most likely the best pure athlete in the NFL, and track provides an expression of that talent.
Furthermore, Rex Ryan appreciates and celebrates sports far removed from the one he coaches. He regularly comments on hockey, boxing, basketball, wrestling (the real stuff), and baseball. It's safe to assume that Goodwin has Rex's blessing in this.