On the south side of Paris (near the Cite Universitaire) in the 14th arrondissement, this park offers a small greenspace with a useful .94-mile asphalt path. Early birds who want to run inside the park must wait for the gates to open at 8am during the week and 9am on weekends. Or, you can run along the park's border after hours without crossing any streets (1.04 mile loop). Jardin du Luxembourg is certainly a closer running option for those staying in Montparnasse, but this park provides another option--albeit farther. Obviously, if staying in the 14th arrondissement, Montsouris will be closer. Bathrooms and water fountains are available inside.
Parc de Montsouris