Sprint the first mile of a long race and the course will collect that debt with interest.

Early intensity borrows from a later part of the effort you have not met yet. It feels like commitment and it registers as progress, which is why it is so hard to argue with at the time. The bill comes due in the middle, where the pace you set is no longer available and stopping starts to look reasonable.

Set the opening pace by what you could hold for the whole distance, not by how fresh you feel. Leave something in reserve on day one of anything long. Finishing is a pacing question more often than a willpower one.