Bring bug spray!!!
We did this last time. Most of the time is spent on things other than hiking. There's the ferry ride to St John & back (that's the longest part), meet the guide, do the hike while stopping to hear what flower or plant we're passing, a stop at the beach, then a stop at a gift shop before the ferry back to St Thomas. If you're looking for exercise, this isn't it. However, the ride to St John was nice, and it was a very easy hike.