I'd like to go through a few thoughts about this.
This is all general advice as it does not fit everyone and in some cases you would need a doctor's or health professionals 's advice to ensure that you are doing the right thing for your body. If in doubt seek a consultation

I have worked as a chiropractor for over 30 years and surfed for over 50.
So lets start;
Joints as a rule are supported by opposing groups of muscles that pull the bones that make up the joint into its various functional positions
To best function they need to be balanced in power across the joint and hold it in its correct positioning during movement.
Prolonged misalignment of a joint or excessive muscle pull on the joint will over time cause some types of arthritus .
Excessive weight, pressure or repetitive usage can do the same or worsen the problem.
Add wipeouts, cold water and being overweight and you can see why surfing enjoyment would be way down.
The challenge, getting fit;
Monday morning at clinic was always about repairing weekend warriors who thought they were fit.
So be realistic about what you undertake.
Believe me if you haven't done it for a while you are not as good or as fit as you were!
Get a check up listen to the advice and follow it, walk at speed heaps, if the joints are not working well get any or all of these, chiro, physio, sports doctor and trainer to devise you a program. ( See that they understand your surfing and how it stresses the body but relaxes you)
Then having done that and spent some money, you will probably like to stop making excuses.
Do your program with some friends, more fun, positive peer pressure.
Drink more water and avoid casual visits to the fridge.
You will probably like to stop making excuses at its white enamel door. Truly, realistically review your eating habits.
This covers fitness, weight, out of breath experiences and each of these improved will make surfing easier and more enjoyable, being over 60 myself I have to keep this in mind all the time.
Joints/ arthritus etc,. Medication first thoughts, steroid injections painkillers, anti-inflammatories,and surgical repairs need to be carefully weighed up by medicos with a risk versus benefit for each individual and their health pattern.
There are joint supplements that can help the joint recover from arthritus.
See a specialist sports physio, chiropractor or osteopath, get the joint checked and possibly aligned , learn from them, shoulder or specific joint rehabilitation exercises, do them as part of your increasing exercise regime.
Very important make your program varied and FUN boredom is a killer of exercise programs and health,
FFS treat yourself with the respect you deserve.
Think about this one too;.
I am a health professional but I went to another health professional/ weight loss program to get myself healthier, because having to face the truth of where I was on weight and health from someone else and have them give me a program meant I couldn't make excuses.
Long post hope it helps. More health means more fun + more surf etc. Surfing more and better now than I was 15 years ago.