We've just got back from a long walk and my 5 year old German shepherd who is usually lively now cannot walk properly on his back legs he was fine until near end of the walk where my partner had to get the car for him. Am I worrying for no reason and he's just exhausted??

GSDs are very notorious for hip, knee and back problems.

Sounds like you need some x-rays.

It could be anything from a "sprained ankle" to Inter Vertebral disk disease to hip dysplasia and knee displasia to him tearing his ACL tendon in the knee.

Because he is what he is (a GSD), I'd go to the vet and get some X-rays... better safe than sorry... Some of those ailments can cause permanent damage to his bones/muscles.

You also might want to check to make sure he didn't pick up a burr or anything like that in the pads of his feet... Sometimes dogs will get things caught in there and limp like they are injured.