A little less than a half hour to get out there and about 20 minutes to get back to Maho.
Ronnie

Drive time from Cruz to Coral is ~30-45 minutes depending upon your driving habits and desire to constantly stop and say 'wow'. Coral Bay to Maho is ~10-15 minutes, again, depending upon your driving habits.
You could hike the Leinster Bay trail to the Johnny Horn Trail and back, taking in Annaberg Ruins. Snorkeling would be at Waterlemon Cay or Maho & Francis Bay. You could visit Annaberg, hike the much shorter Francis Bay Trail and then snorkel in the same general area. This would leave you enough time to wash up and make dinner in Cruz Bay. A full day, but certainly possible.
See this site for a map to give you an idea of location, etc.

http://www.varlack-ventures.com/vvfiles/vi-beaches-hiking.htm

Thanks everyone! Will have 3 days on STJ, then 3 on STT. Hoping for plenty of outdoor time. Running shoes sufficient for most hiking? Can I get by with Tevas? Thanks!

Not familiar with Teva brand, but a good quality runnung shoe will be fine. Make sure you carry water and use sun screen while hiking.

If you are planning on 3 days on STJ and want a really nice hike, look into the parks department Reef Bay hike. They drive you from Cruz to the trail head on Centerline Road, escort you down the mountain to Reef Bay, stopping at the Petroglyphs and Sugar Plantaion ruins along the way. You can snorkel in Reef Bay and take their ferry back to Cruz Bay.
Takes about 2-3 hours and is beautiful.
You can check it out at the parks department in Cruz Bay,
Hang a left from the ferry dock, just past and opposite Mongoose Junction.