Meijo Park is a small, but centrally located park in Nagoya that is pleasant for running. There are numerous paths and recreation facilities in the park. There’s a marked 1 km path around the park, which can be added to with some of the other trails. Bonus: there are locker/shower facilities.
Longer Run: You can run on the grounds of the adjacent Nagoya Castle. (Admission required, limited opening hours). Best to do it off-peak hours.