At 148 acres, this is the third largest park in Rome. It is centrally located and makes for a great run. It’s possible put together a run of about 3 miles along the paths of the Villa Borghese. Sites include the Villa Borghese Gallery, lovely gardens, Museum of Modern Art, boating lake, and Piazza Napoleone. The paths are wide and gravel, and fairly shaded. Due to its central location, Borghese can be accessed via the Piazza del Popolo or the Spanish Steps/Porta Pinciana, among other options.
Good add-on/combination run: Villa Ada, another large park in Rome, less than 2 miles away. Use Via Salaria or Viale Romania. Can also add on with paths along Tiber RiverGuided Walking Tour of Borghese Villa Ada Running Guide