Developing your strength, balance and flexibility is key when you want to first learn to surf or improve your surfing technique. Yoga and Pilates will improve your strength for paddling and pop ups, balance for better rides once you're up, and flexibility to lower the risk of injury.
Your general level of fitness and energy will really show in the surf. To help you to improve your confidence in the water, explore your body's ability to move, to build up stamina and to release short burst of energy, through running. Just be careful to find a healthy balance to how often or far you run, and invest in good running trainers.
For a structured class environment where you know that you will improve your strength, stamina and general fitness levels, circuits is perfect. Make sure you keep it interesting by trying different classes to keep your motivation up.
Pure fun- you will hardly know that you're working out. Breakdancing and Brazilian Capoeira not only have you sweating from the cardio involved, but will build your core strength, flexibility and improvisational techniques- ideal for the ever changing waves you'll be surfing.
Here's a little Capoeira inspiration for your day, tune in at 50secs to see why Capoeira is such a good work out.