The one above is a 40mm Bofors anti-aircraft gun from the Second World War. Until very recently, there was still (corroding) live ammunition racked up on the gun! There are many other larger guns atop the cliffs of Steep Holm, dating from the Victorian era. All were intended to defend approaches to the large ports of the upper Bristol Channel.
Great colours and setting for your pictures. Many years ago I slept in beach hut at Studland until an unexploded bomb was found underneath it. You have a real challenge in preparing for your ultra marathon, not least in time spent putting in the miles. The stamina to keep going for that distance depends on the ability of your heart to keep supplying your muscles with nutrition and oxygen, in other words cardiovascular output. If you want to build stamina more quickly how about interval training? If you run with a good style and speed then walk to recovery once you start plodding then run with speed again you will find that your stamina improves. Even alternating sprinting with easy paddling in a kayak will build your stamina and indirectly help your running because of the improvement in cardiovascular output.