A spokesman for the Royal Sussex County Hospital said the man died at 2.30pm.
England cricketer Matt Prior had officially started today's race, the fourth Brighton marathon, in Preston Park at 9am.
Organisers said 9,315 competitors were taking part.
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A mother has appeared in court, charged with the murder of her three year old daughter in Hampshire. Tests found she'd drowned at her home.
The Duke of Cambridge said the new prince was "behaving himself" and sleep was "going reasonably well so far".
The challenge came as Theresa May vowed no stone would be left unturned in putting right the status of the Windrush generation.