The most common ideology with regards to staying fir for surfing, is that you have to surf to stay surf fit. That nothing can keep you fit for surfing except the actual act of surfing itself. This is not true. There are very effective ways to keep fit for surfing without paddling out. Here are five of them.
1. Running.
Grabbing your running shoes and heading out there is a perfect way to keep fit for surfing and to keep the weight off.
Many surfers, however, complain about bad knees and that they can't run on their knees. They should then look at...
2. Swimming.
A hard work out in the pool is actually way more beneficial than actually going for a surf, it's just not nearly as much fun. Swimming keeps your lungs strong, your body fit and your mind alert. It also doesn't apply any stress on any joints - you don't get bad knees or sore ankles from swimming. The only real problem with swimming is that you need a heated pool in winter to keep going. Here are a few simple swim training programs to get going 
3. The Gym.
One of the best ways to keep your body supple is to do some...If you decide to keep fit for surfing by going to the gym it is imperative that you know and understand your muscle groups, and what works for surfing and what doesn't. If you're not completely sure, then find an instructor who surfs. he or he will be able to guide you.The last thing you want to do is work some muscles that are going to hinder your surfing fitness when you get back into the brine.
4 Yoga.
As surfers get older, so their bodies get stiff and less flexible. This makes for strain in the water, and often leads to injuries. One way to compensate for this is by stretching, but when that isn't enough then it's time to hit the yoga mat. .
No need to get to serious about it, but the gentle stretching action of yoga on a consistent basis will keep your body more supple and less injury-prone so when you get back in the water you have less chance damaging yourself. Finally, if you're feeling a bit more confident, and keen to get to your upper fitness levels, you can always try...
5. Crossfit.
A fairly recent phenomenon, CrossFit is a program developed to offer a full-body workout that combines elements of cardio, weight lifting, gymnastics, core training and more to prepare the body for the unexpected. It's hard and it's intense, but it'll most likely make you fitter than most surfers who haven't even had a break from the water. The only problem with Crossfit is that it can get so intense that people can open themselves up to injury while training. If you get through a Crossfit training program and come out the other side, you will most likely be that person in the water, even after a long break, who will be catching the most waves, and there's nothing wrong with that. 
The most important thing to consider when out of the water for a long time, is that surf muscles will quickly disappear, so it's important that you keep aware of your fitness and don't let it all go to pot. If you leave your fitness and have the ideology that you'll soon be able to pick it up when you're back in the water, you're wrong. It'll take longer than you thought. Way longer.