Start while the details are fresh
Record the event name, date, distance and the source of your official result. Note the weather as you experienced it, where you started and what surprised you. Keep observations separate from explanations you are only guessing at.
Remember the practical details
Was arrival straightforward? Could supporters find you? What did you wish you had packed? Those details make the note useful the next time you consider the event. Describe your own experience without assuming that everyone had the same day.
End with one thing to carry forward
Choose one practical lesson rather than making a list of everything that could have gone better. A well-chosen meeting point or a calmer journey can be worth repeating. Leave room for a photograph, a ticket or the name of someone who made the day special.