What I've found so far:
1) 3.8-mile loop at Sharon Woods, plus the 0.2 loop in parking lot to make it an even 4. (MapMyRun is awesome, as I don't have a GPS watch and don't track mileage that way). I map out my maps and then memorize the routes when I run them.
2) Antrim Park, plus the 3.5 mile segment up to Worthington Hills Park and back (7 miles). The loop is only 1.18 miles so I run a little bit under the bridge to top it off.
3) Alum Creek Trail between Westerville and ODU--it's a bike path with tons of shade and neat bridges! There's also a random spur that has a bridge by a greenhouse that goes nowhere. Cbus is weird sometimes.
4) I haven't done Blacklick or Battelle-Darby but those parks look ginormous and good for running. Highbanks has some good trails but they go all over in weird directions and it's not as straightforward.