If you're not up for the challenge of an Ironman triathlon, which involves swimming 2.4 miles, biking 112 miles, and running a full 26.2 mile marathon, tackle a much shorter and more doable sprint triathlon which includes a .5 mile swim, 12.4 mile bike ride, and a 5K (3.1 mile) run. The distances may vary a bit from course to course depending on the body of water for the swim and the terrain for biking. Just be warned — a sprint tri is the gateway race to longer and longer distances.
Here is the training schedule, created by the YMCA Tri Club, that I used a couple of years ago when training for my first tri. It is a great training program that slowly builds so it is great for beginners but has easy recovery weeks every three weeks or so. Even if you're not planning on participating in a triathlon, this training schedule might be just the thing you need to get ready for the beach. You can choose to do one workout on each day of the week or double up on workouts to give yourself a rest day.