If I had to give my younger self some career advice, it would be this: work smarter instead of longer.
The world, your bosses and HR managers and clients, will conspire to get you to prove yourself by putting in long hours, but working long hours often has a counterproductive impact, unless you are a factory line worker. Eventually, you end up tired, in a rut, and repeating the same barely logical patterns.
"Self," I'd tell myself, "sell your talent, not your time."
Case in point: my best articles generally take the least time to write. My most inspired and popular pieces fall out of my brain so quickly my fingers can barely keep up. The reverse is often true; my worst, least inspired works take forever to produce. When it's a struggle to write something, the odds are pretty good it's also going to be a struggle to read it.