Deciding to get fit and doing my first exercise session is not the same. Between deciding to start, and starting, there is a journey.
In this way the Start becomes a subversive Destination. And you can get lost easily on that unseen journey or get distracted and never get to the Start.
This is helpful: Right after you have made a decision - also decide on the clear and concrete first step.
Get to that destination, that is the start, in the shortest possible time.