Well, if you can't get out to surf or SUP, mix up your fitness activities:
Weight lifting: Lighter weights with higher reps (10-12) to increase stamina. You want to maintain flexibility, not bulk up. Work muscles in both directions to include opposing muscle groups.
Body weight exercises: pushups, pullups situps, dips, burpies, squats.
Stretching and flexibility exercises: Of increasing importance as one gets older.
Swimming: Open water or pool.
Running: Crank off sets of pushups throughout your run.
Maintain a good, balanced diet. Also of increasing importance as one gets older.
SUP, Kite, Skate, Swim and Stretch. The other day i surfed, kited and sup'd all in the same day. Each activity is similar but focuses on separate muscle groups.
Long distance SUPing is the newest thing i have been into and it is an amazing workout. A few weeks ago i paddled from one island to another, almost 6 miles.
Swimming is by far the best "workout" to cross train for surfing. Running has never been my thing and is actually really bad for your joints so i tend to stick to water activities.
If you pay attention to the Pro surfers warm up before heats you can pick out some great stretches specifically focusing on surf muscles.
I do cardio and weights. I had to stop running a long time ago, and I really don't like pools, so I have to do cardio on the machines at the gym three seasons out of the year. I think there's been some good advice here already, but your best bet is to find a personal trainer that surfs to get you started. I did a few sessions with one many years ago, targeting surfing-specific routines, and it's kept me in the game as I've gotten older. (Who am I kidding... "old!") I don't have time to go more than about 3 times a week, but ideally a daily workout is much better.
I have been a long distance runner since I was 13. I am now almost 31 with no joint issues. In fact, the only times I feel real joint pain is after consecutive days of surfing. It is easier to overexert yourself fighting currents or even riding a wave (push, pull, crouch). I am not a slow runner either. I average 7 minute miles and run about 5 miles a day.
I have been using my son's ripstik. It is fantastic and has really helped my surfing. I also swim 3 days a week.
if you're not a program guy, check out the book "Fit To Surf"
It gives you 100+ exercises including stretching, yoga, indo board, and paddling. It gives you pictures of the exercise and the muscles it work along with what they do. In the back of the book, there's a bunch of sample workouts and pre-made programs for home workouts and the gym. It's pretty neat
Bench unless you can't bench 135 then just stay home. Great waves here you should get out and catch some barrels
We all know someone who is a runner and has joint issues. Surfing has very little impact on the joints unless (like you said) if you have bad form
yoga and more surfing. Surfing in the winter with 10 lbs of wet rubber on is like swinging a baseball bat with a doughnut on it.