I've always found that working significantly more than average is a mental thing more than a physical thing. It can become a bit of a habit, not easy to get int

I've always found that working significantly more than average is a mental thing more than a physical thing. It can become a bit of a habit, not easy to get into, but once you're in the rhythm, "it just works". The signal is probably not so much the raw energy level, but the discipline to focus that energy on a task. If you choose to work that hard at 20 instead of spending half your day hanging out with friends, that might be a good signal for how hard you'll work when you're 40. It is in my personal experience, but that doesn't say much.