The power of persistence
In the words of Napoleon Hill “Patience, persistence and perspiration make an unbeatable combination for success”. Whether it's finishing a couch to 5k, smashing a distance swim or nailing an enduro challenge, the 'three Ps' (emphasis on the perspiration) are a simple reminder to keep moving forward.
Chip away at your goal day by day, week by week and don't forget to reflect on how much stronger or faster you have become since you started out. I find that keeping track in a journal or looking back over fitness tracker stats really helps for this.
Sometimes we get so caught up in being impatient (guilty!) that we spend more time focusing on how far we have to go, rather than reflecting on how far we've come.