Lianne's advice is spot on.
Our story: My husband and I have sailed all our lives on lakes. We chartered once in the BVI's in 2000 with a training captain. Fast forward to 2014 - we spent 10 days on a cat in the BVI's getting our 101, 103, 104 and 114 (through Virgin Island Sailing School out of St. Thomas). March 2015, November 2015, and April 2016 we've chartered boats by ourselves in the BVI's. The classes gave us the knowledge and confidence to do it ourselves. Strongly recommend them if you are at all questioning your ability.
Fast forward to May, 2016 - we bought a 53' Carver Voyager* and plan to do the Great Loop when my husband retires. :-)
*not a sailboat, but lots of our ASA training applies to big boating

PS: TMM is a great organization!