"I recall a disagreement I had with a friend many years ago. My claim was that anything I set my mind to was achievable; he thought this idea was ridiculous."

You can do anything you set your mind to if you have infinite time (and money/other resources). As a thought experiment if you are an immortal billionaire with perfect health, you can do anything you want.

Fitting a plan (to successfully complete audacious goals) into a context of limited resources is the real challenge. This is why startups talk of 'runways' and scrounge for funding.

(that said, breaking things down into smaller steps is useful)