North towards Cincinnati
From the downtown bus hub on Vine Street find a bus going towards Newtown Pike. Take it as far as possible to the on ramp of Exit 115. This is the northern most exit along I-75. Reject any rides that say they can take you a few miles up the road where I-75 and I-64 split. It will put you in an uneasy place along the highway where cops pass quite often. But if they are going towards Georgetown then you are good. That is on the way to Cincinnati.
South towards Knoxville
From the downtown bus hub take a city bus going to the Hamburg Pavilion where a lot of restaurants and stores are. This is at the end of Man O' War Boulevard where there are ramps to get on I-75. There is a lot of traffic getting onto the highway from there and within minutes you should be on your way towards south-central Kentucky and Tennessee along I-75.