The Sanford Riverwalk is nice, but awfully short. I've walked it three times and am going out again today.
I love the trails at the Spring Hammock Preserve, also in Sanford, on the bottom edge of lake Jessup. Great swampy terrain, giant cypress trees, lots of birding, and long elevated boardwalk trails.
I did some short hiking at Kelly Park (Wekiva), walked along the pretty sections of the Cross Seminole Trail, and will be checking out trails at Lake Jessup Preserve soon.
I've heard walking in the Econ is pretty good.
All the walking here is pretty flat!