Start with the planck length. Not, how long you can hold a plank, but rather the smallest length you can possibly go to on a fitness journey. I read Atomic Habits recently and James Clear talked about how success is best made from many continued 1-2% improvements. You don't need a habit of going on a run, you need a habit of getting out of your chair and on your front porch. He talked about a man who committed to going to the gym for 5 minutes. He diligently made it to the gym, did precious little, and left for a long time. Eventually the rest took care of itself because this man started identifying himself as someone who doesn't miss a workout.

To that end, proper, fully fleshed out diet and exercise lifestyles are tough. It's a confluence of skills and a transcendence of environment. Think of fitness more like learning to play the piano. You wouldn't necessarily start by learning songs - you need to know the difference between black and white keys first. Start with skills like getting up out of your chair and standing on the porch, not even taking a walk. Maybe find a college level nutrition book to read about macronutrients. Maybe learn how to read nutrition labels. A bunch of skills like deciding it's time to work out, planning meals in advance, having a workout plan, and knowing how to track progress will develop over time, but they're made from 1-2% gains.