If Nike has "just do it," then the Chinese has a proverb along the lines of "catch a faster horse on a running horse." This saying that progress is incremental can be applied in many contexts.
One example is, while you're applying for your dream job, you should already be working a job. Waiting for that dream job alone isn't enough.
Another is, if you want to be an artist, then start drawing. Not just when you have the time.
I think too many of us like to put off our life goals because we think we need to block large chunks of time to commit to those endeavors.
But the reality is that whatever we spend our free time on is already likely our passion. If you love television and YouTube, chances are that you have a keen and empathetic eye for storytelling. If you spend your time fixing things around the house, then you have an engineering mind. If you like to doodle, then you're the type that likes to put stories to paper.
Finding a horse involves asking yourself, "where do I want this passion to take me?" Once you've done so, get on it, and one day you'll find yourself a Secretariat.