In five months I have gone through three 100 page sketchbooks and I have had fun and reached flow state pretty consistently the whole time through. The first of the sketchbooks was basically just drawabox exercises up until the 250 boxes challenge. At this point I transitioned away from drawabox as I was having enough fundamentals to have fun and make progress on drawings I cared about.

All these resources and books may seem overwhelming and you definitely don't need to go through all of this material. The key for learning drawing is to figure out what is exciting for you to draw and get over that first hump to get to where you can draw something you see have potential even if it is not great. That's when the momentum builds and it becomes self propelling. Drawing is basically a language for communicating 3d form on a 2d surface, and like learning Spanish the key is just to get enough of a vocabulary that you can get started conversing. The rest follows once you can talk a little.

If you're savvy, you can probably get ahold of the course books for the self-paced "Famous Artists School" courses from the 50s. They later became the "Art Instruction Schools" courses that had the commercials with that dorky guy saying "Do you like to draw, or paint, or maybe just sketch and doodle?" It was created by Norman Rockwell and Albert Dorne specifically to guide people to teach themselves how to draw. You obviously can't get the mail-in critique, but if you have the discipline to do all of the exercises, you'll still do well.

NerdForge recently helped her boyfriend learn to draw from scratch, and she lists all the stuff she used to help him get there, and you can watch him go through parts of them as well.