Start from a realistic effort plan, allow for conditions and avoid turning the opening kilometre into the whole race.
Choose an effort you have rehearsed
Training gives you a range, not a guarantee. Base your opening effort on recent sustainable running and adjust for distance, hills, heat and wind.
Crowds and adrenaline can make the start feel unusually easy. That is a reason to stay controlled.
Break the race into decisions
Settle early, review effort through the middle and only increase late if you remain in control. Walking an aid station or steep section can be a deliberate pacing decision.
A watch is useful context, but breathing, form and symptoms still matter.
Stop for warning signs
Chest pain, fainting, confusion or severe unexplained breathlessness needs urgent medical attention. Do not continue with severe pain or an inability to bear weight.