Progress is like a diet. I'm in the design world, so I am able to see progress with my eyes. (Note: design isn't just aesthetics)
If you are generally a healthy person and you're trying to lose weight, you know you have to hit the gym hard. You opt for the smoothie over ice cream. Fruit becomes a treat. You rarely splurge, so when you do, you know it's going to be the best damn cupcake in the state. Time + good habits + baby steps will make a difference, but the harder you go, the faster you will hit your goal.
Why are our vocations or hobbies any different? Perhaps some of us are healthy and in no hurry to improve. Maintaining the same pace will inevitably lead to expertise years down the road. But when I see people like Jon Contino or Jessica Hische going balls to the wall to perfect their craft, I realize that why everybody fawns over their work. They weren't born with a magic gene; I think they just work like mad dogs. They have young faces and old sensibilities.
The book constantly has me evaluating my rhythm and flow. I never ever thought it would be this much work, but it's 100% worth it. I look back on the earliest drawings and I see a completely different illustrator: charming, but not stretched. It's amazing to know that when the book is finished, I also get to keep the best version of my craft. I have so much to learn, but what a beautiful thing practice is. Moving in fast forward was the boot camp I didn't know I needed.
*Photos of Swan Lake by White Worm