After MU doing his thing, The Doctor told us three to do something, something that included carrying a bucket and finding a lake in the middle of a forest but do it, you would have to do it really fast, running really fast.
So with that Castiel picked up one of the three buckets that were on the ground, and Castiel started to run towards the forest, and looks for the lake, which he find about 3kms into the forest, he fills the bucket with water, and proceeds to run back to The Doctor, Castiel makes sure he runs along all types of surfaces, narrow, even, uneven etc. Returns back to the The Doctor and tips the bucket of water out somewhere near The Doctor, Castiel then rise and repeats said process for many hours, and many days without resting, running fast, and running on uneven surfaces. As Castiel progress' both of his arms are now rock hard, as well as his legs due to the running, as time went past. Castiel got faster and faster and returned much more sooner from the lake each time.
"I think I am ready for the next part of the training. I don't feel too tired, I am pumped."