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 teenager from Portsmouth who compared himself to serial killers has been jailed for 21 years for the attempted murder of three women.
Latest figures reveal nearly two million unsafe or noncompliant products were identified between April and December 2014.
Friday's forecast for the west of the region