FWIW, getting across the river should be easy - there are lots of commuter cyclists downtown. I'd worry more about getting across 270. When I lived in Columbus in the mid-80s those were lightly traveled crossings, gateways to rural roads; now, the whole area west of 270 is packed suburban sprawl and the overpasses and on/off ramps have a lot of traffic.

COP has regular rides out of Grove City, Dublin and Hilliard, and any of the ride leaders would probably be happy to help you with routing. Here's a link to their ride schedule - if you click on the ride, the ride leaders' contact information will be there.

MORPC seem to have their stuff together, though I haven't actually used any of their maps. I'd be a little leery of mapmyride - I know there are routes mapped closer to where I live that I wouldn't feel very safe on (and there are plenty of safer alternatives).