You are not supposed to run in the Arboretum. Trails are designed for hiking only.
The Inner Loop is - surprisingly - a loop trail inside of the Outer Loop
. The trail starts just south of the visitor center and has a dark green blaze color assigned to it. It passes near some lovely swamps and ponds and goes through a variety of different plant life. Most of the trail surface is wide singletrack (about three feet wide) made out of wood chips, so you could push a stroller along but it will be a little more work than the Outer Loop
. The Inner Loop also has a large number of boardwalks so you can keep your feet nice and dry the whole way.