You are not supposed to run in the Arboretum. Trails are designed for hiking only.
The literally-named South Meadow Trail is the southern of two trails that cut across the meadow in Houston Arboretum. The eastern half is in the meadow and along the treeline, while the western half is actually more of a pond trail. Near the western end is a large wooden platform that juts out into the pond and offers nice birding opportunities. Like it's northern twin, this trail connects the Inner Loop
and Outer Loop
trails so you'll have to take one of them to get to it.