Go look up the
Beaufort scale, and begin to understand the ocean.
The fact that some were that long doesn't mean much when you realise how little mass they can lift.
Okay, so the 12lb gun weighs 500kg's, and the graf zeppelin has a useful lift of 10,000kg's.
Lets say you mount four (a mildly useless number), thats 2000 kg's.
Each shell is 6 kg's. Lets say each gun has 50, thats 1200 kg's.
That gives you only 6,800kg's of payload for bombs. Thats just Seven 2000lb bombs. A thoroughly useless number.
So you see you get a pointless zeppelin...