One hot day as old man Stone sat solemnly on his front porch he noticed someone walking across the old bridge. It startled him to his feet and before he knew it, he was running towards them.
Actually, it was more like a fast pace that would prove too much for his heart. No one was allowed to walk across the old bridge. Everyone knew that. Just like everyone knew it was Stone’s responsibility to keep others away.
He clutched his chest and fell to his knees. Running was never his strong suit. Not like protecting all of humanity was.
I would ditch " one day while" in the future. You can use 3 words more powerfully elsewhere, I promise.