You are not supposed to run in the Arboretum. Trails are designed for hiking only.
Outer Loop is a descriptive name of how this trail features in the Houston Arboretum. However, the trail doesn't quite close the loop - you'd have to take the Alice Brown Loop
for a short distance to complete the loop. The entire trail is very wide and solidly made of gravel fill. Thus the trail is completely accessible. There are a number of benches along the trail as well. The trail has a nice mix of shade and sun, as well as forested regions, a few meadows, and ponds.