The core of this idea is one that Chris Williamson has talked about a lot. Many of us live day-to-day thinking that the life we're living is just something to get through on the way to our "real life" where we'll have all the things we want and be all the things we want to be. This is a fantasy designed to keep us from the existential anguish of watching our actual life slip through our fingers. But this is our life. Whatever you're doing right now, that is your life.
4000 weeks.
That's all you get. Stop pretending that someday your life will actually begin.
The clock is already ticking and has been since you were born.
That's why my woodshop is my 2nd apartment bedroom. That's why my gym's in my living room. I'm not putting my goals on hold just because my circumstances aren't perfect.
My high school marching band director had a saying: "Practice like you want to perform." This could be taken literally as "don't screw this up in practice or you'll screw it up in perfomance." Or it could be taken in a more meta direction as "the dedication you show in practice will translate as competence in performance."
Unfortunately there is no practice for this life. We have to do this thing for the first time live and as far as we know we don't get to do it again. So perform. Even when you're nominally practicing. If you're in school or in training for work, perfrom. You're learning, so you'll make mistakes, but embrace the grind and make it something beautiful.