Starting a marathon at a sprint is how you meet the finish line in an ambulance.

Enthusiasm at the beginning is the most common way people end things early. The pace that feels good on day one is set by excitement rather than by what you can sustain, and the collapse that follows gets misread as evidence that you were not suited to it. The problem was the opening speed, not your capacity.

Start any new effort at a level that feels almost too easy and hold it for a month. Add slowly, and only when the current level is boring. Finishing at all is worth more than starting impressively, and the two are usually in conflict.