The Riverwalk is the running highlight of downtown Jacksonville. There are pleasant paths for running on both the north and south banks of the St. Johns River, connected by the Main St. Bridge and the more pedestrian-friendly Acosta Bridge. The North Bank of the Riverwalk is about 2 miles, from the Fuller Warren Bridge (I-95) in Brooklyn, east to just past the Hyatt. The South Bank is about 1 mile, from the Acosta Bridge east to Broadcast Place. A full loop of the both sides, plus a loop out to the Baseball Grounds & Everbank Field complex is 7.7 miles, though the route can be adjusted for distance preferences.
Start at Jacksonville Landing and run west to the Fuller Warren Bridge (2 miles out and back), then cross over the Acosta Bridge to the South Bank. Coming off the pedestrian path, run back toward the water, passing the Mosh Science Museum and Friendship Park/Fountain, then east along the South Bank Riverwalk path to the end (1.3 mile out and back). Cross back over the Main St. Bridge. You can end here (5 mile mark), or add on 2.5 miles by running east for 0.5 miles to the end of the North Bank path, and then do a loop of the Baseball/Stadium complex, returning through downtown streets (Duval, Liberty) to the start.