I recently borrowed a mate's 7'2" to try to get back to riding a shorter board than my usual 9'1. Ive used it a few times but each time Ive ended up with a really sore lower back. I was thinking maybe its because the weight of my legs isnt supported like it is on the longboard, and my aging back cant handle being arched at the head end and unsupported at the feet end. In which case I'll probably give up on shorter boards for good. But before I do just wondering if anyone else has noticed and/or fixed this.

I have back issues as well and found the same thing. I have a 7"2 hybrid which is a great board but I occasionally got a sore back. It may also have to do with how much flex your back goes through as opposed to a longer board which lets you lie a little flatter. Pherhaps the extra effort needed to push along on a shorter board throws the back out a little bit?

Either way im stoked with the longboarding now, so much enjoyment and a relaxed lifestyle that I may have not discovered for a while had it not been for my dodgy back

You have a good point Kearnsy, I have been through all the treatment for a bulged L5 and it's manageable given the proper stretching and not overdoing the impact. I will admit that I dont stretch much before I surf and should do to be honest.

The exercises you mention are the same ones I use on an almost daily basis to keep my lower back all good so it does make perfect sense to do so before a surf. I just find it really hard to when im so amped for a wave!

Thanks Mac, I do my best

You can also try putting a hot water bottle on you lower back if you have to drive to the beach. This also helps get warmed up quicker

Interesting MickJ . I have found completely the opposite. I tend to get more back pain when i move to a longboard after surfing a shortboard for some time. My body is more parallel with the water on a longboard so to see where i am going, i have to lift up my head using my neck and backmuscles. When i am on a short board, the tail of the board is somewhat in the water, giving my front end some elevation. The angle of my back lift is less acute than with a longboard.
I do find too that when i scale down board size, I really notice the lightness and looseness of the shorter board. Turns and manoeverability are so much easier. Because of this my back needs to be able to twist laterally more without pulling anything. Because there isnt much lateral movement we participate in during our day, we need to make up for that doing stretches. Yoga postures that twist from the waist are brillliant and will keep our back supple and flexible. Even sitting on a chair with your thighs parallel to the floor, twist gently from the waist. If you can reach the back of the chair with your hands, use it to hold the position. Lead from the shoulders and keep your back straight.
Stretching before you surf is always a great idea, doesnt matter how daggy it looks. The older we get the more important it is!! Hope this helps. :-)

If you want to know what daggy looks like, take a look at me when I get a cramp, now that's funny.

It happened on Saturday which is a bit strange because it's more of a winter thing.

Usually it's the same modus operandi;

calf mussel on the left side
cramp starts as I'm paddling for the wave
I stand up thinking it will go away
Fall off side of board looking like a complete and utter spaz

It's called gettting older. Get use it, parts of my body that never hurt before are now beginning to give me grief and trying to do the things I did a few years ago now hurt even doing streches and warm ups. Getting old sucks!!!!