Start while enthusiasm is useful, then build something stronger than enthusiasm.

Enthusiasm is a good launch fuel and a poor engine, since it is unreliable and disappears exactly when the work becomes tedious. Using it to start is sensible. Depending on it to finish is the most common reason projects die around the middle.

While the energy is high, build the structure that will operate without it. A schedule, a commitment to someone, a routine. That structure is what carries the project when the feeling has gone.