Updated: Nov 14, 2020
Besides, starting is only a single piece of the puzzle. We all know that gyms are full in January, and slowly empty out by March. Once you've figured out how to start you need to figure out how to sustain, and that takes deeper change than simply taking the first step. That requires a plan, and a willingness to stick to that plan. So far, it seems like the key to sustain is habit formation.
An ancient Greek poet named Archilochus once wrote (probably on some papyrus or a stone tablet): “We don't rise to the level of our expectations, we fall to the level of our training." James Clear, author of Atomic Habits, modernized this a bit (on Twitter obviously):
Meaning that setting the goal of going to the gym, of losing weight, of running a marathon, or even taking the first steps towards achieving any of those things is a very small part of actually doing the thing. Keeping on this marathon metaphor, it takes maybe 2 two seconds to say out loud, "I'm going to run a marathon" (try it at home) and another 5 minutes to go on that first run.
At the quickest, it takes about five runs/week for 12 weeks to get in shape for a marathon. For argument's sake we'll say 30 minutes per run, which adds up to about 1800 minutes total spent running. those first five minutes and two seconds make up about .0028% of the total time it'll take you to get ready for a marathon! This is a clear sign that as quick as you should be to start whatever challenge it is you want to start, you need to account for the other 99.9972% of the task, and that takes a system to sustain.