If your thinking about surfing then you also need to think about strengthening your core. Surfing is a full body exercise. Nothing compares except lap swimming, which is excellent and should be included in any exercise routine. I have found a combo of land and water works. Here is a land work out that does the trick and makes me sweat.
Squats...everyone knows this right?
Push ups...do them on your knees to get started if you have to. Make sure your hand placement is inline with your chest and bend your elbows. Go as low as you can and still be able to push up, build your strength and go lower until you're barely touching the floor and push back up.
Pop ups: These are like Burpy's, why burpy's who knows?
Step one: squat, lean forward and place hands on floor, kick your legs back and out, put yourself in a push up or plank position.
Step 2: Lower yourself to floor and adjust your hands to be parallel to chin above shoulder and the other hand in plane with chest. Which hand goes where? You will decide based on if you like your front foot to be your left or right. I am a "normal" foot ie; My left foot goes in front. If you are left foot front then you are goofy foot so switch the hands.
Step 3: In one fluid motion push up with your hands and use your toes quickly bringing your front foot forward, leaving your back foot in same general space. Using momentum, place your front foot forward while turning your body, the direction of your back foot. At this point you should be on your feet facing rt or left depending which foot stay back.
Step 4: Stay low and rise midway. repeat. Your feet should be shoulder width and inline with each other, back foot should face the same way you turned and your front foot should be turned as well perhaps not as much. For the most part, your back foot steers your surfboard.
Get started with 4 sets of 10 squats 10 push ups and 10 pop ups once or twice a day. I like to to do the squats, then push ups and then pop ups ride the bike or run stairs at gym or do a set of weights for ten mins then repeat 3 more times. Do this for 3 days rest and start routine at a count of 15, 3 day's add 5, now you are at a 20 count, go 5 day's and rest 1 add 5 more squats push ups and pops.