Salt Fork is an amazing park with nice lodge+cabins, and the surrounding area brilliant for riding (and run/swim). You should go see "The Wilds" while you're there, closest thing to an African safari I've ever seen

Also the Hocking Hills area is amazing. Tons of running trails, great roads to bike, tons of cabins for rent, and tons of natural sites to see

Hocking Hills - http://www.heartofhocking.com/Hocking_Hills_Map.htm

Hills won't be a problem, in that area you won't find anything flat, very hilly. I never swam at Salt Fork, but used to do a lot of skiiing there. (used to live about an hour northwest of there) I know they have a swimming area, so would expect you can find a place to swim. The lodge at Salt Fork is great. Should be some trails if you want to do any trail running.

Most of the county roads should be good, safe, light traffic. Not sure about the conditions of the roads after a tough winter. Rt. 250 I would expect has lots of truck traffic, a bit further north than the area you mentioned. Rt. 258 from Newsomerstown south and east is awesome. Rt. 800 going south of I-70 should be great, some traffic, but really gets into more remote areas. Most of the state roads south of I-70 should be light traffic, very hilly and remote, the county roads even more so. As mentioned, anywhere near the Hocking Hills area is totally awesome! Most of the area from I-70 south to the Ohio River from Columbus to Wheeling is very very hilly. Seneca Lake should also have swimming, as would Clendening, Tappan and Piedmont.

If you do end up at Salt Fork Lodge and are looking for some trail running, here's a link to a 10 mile route that I think is pretty permanently marked. It's used for a summer and winter 50k every year. Hocking also is beautiful and full of hills although the trails around there are better suited for hiking than for running. Hocking does have lots of touristy type sites to visit.

Based on what I've seen of each area, Salt Fork would be good for swimming because they have a large lake but I haven't seen the same around Hocking.

In terms of restocking and eating, Cambridge is the nearest town to Salt Fork and Logan would probably be your best shot for Hocking.

And if you are interested in this sort of thing, Hocking has some of the best dark skies in the state for astronomy viewing.