One hack for forming new habits is to commit to activities so small and seemingly insignificant that they rule out the perception that you don’t have the time or energy.
Do just two push-ups. Floss one tooth. Drink one extra glass of water per day. Meditate for 30 seconds. Stretch one muscle group before bed. Writing a 365 page book is daunting (I’ve done it), but writing one page per day for a year isn’t.
Don’t be concerned with volume. The goal is consistency and continuity. Most people find after a few days that it’s easy enough to do more push-ups, floss all their teeth, drink far more water, stretch more, and are well on their way to authoring a book.