The official park web-site definitely has the lengths of the trails, and grades the difficulty of each. I assume you're looking for something more in-depth than that??
Note: the above link should be for the trails. Not sure why the web address indicates "shopping".
It's been many years since I visited Hocking Hills, so not really able to compare. I would say that the hikes at Starved Rock are fairly easy. The only difficulty you may face could be muddy trails and a few areas with many steps.
Are you looking for some easy trails? If yes, consider parking at one of the lots on the far east side of the park. From there, hike back to Illinois, Kaskaskia, and Ottawa Canyons. You will also see Council Overhang here. For another short hike, park at the lot off Route 178 and hike back to St. Louis Canyon.
If you are at the Lodge or Visitor's Center, you can take a 3 mile RT hike westward towards St. Louis Canyon, seeing Aurora, Sac, and Kickapoo Canyons along the way.
If you hike east from the Lodge or Visitor's Center, you can make a nearly 10 mile RT hike all the way to the far eastern end of the park. Or, you could do a short hike or something in between. Look at the map in sola's link and make sure you do a loop - follow the bluffs one way, and stay down below the other direction.
If you have time, visit Matthiessen State Park, which is just down the road from Starved Rock.
At this very moment, the vast majority of trails at Starved Rock and Matthiessen are closed due to flooding.
Having said that, we stay/hike in the area several times a year, and wouldn't classify it as 'flat' in any fashion. I guess it sort of depends on what your definition of 'easy' is. While many of the trails have some areas of wooden walkways and stairs, there are MAJOR staircases on several of the routes. The vast majority of the trails without major staircases still have lots of hills to climb up and down. I personally wouldn't call any of the trails at SR 'easy,' nor would I call them difficult. What I can say with great authority, however, is they are by no means flat.
I'd be happy to answer any more specific questions you might have if you'd like to message me.
I think they are flat and easy
I would say in general that the trails are moderate.......if you just hike a couple of them it's pretty easy. But there are enough trails there to spend a whole day hiking, and after a day of hiking there, we are pretty tired. Plus there are some stairs which can really tire out the legs!