Yes It’s endorphines. You feel strong and refreshed because you are, stronger and more refreshed, that’s your body talking, and it knows what it likes. You can get addicted to feeling strong and working out, just as much as you can get addicted to feeling lazy and doing nothing. I say get addicted and give your body what it wants. I’m getting back into it, and when I feel that way, after working out on the beach, I wonder what took me so long.