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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Thursday morning's weather for the east of the region
Thursday morning's weather for the west of the region
South East Coast Ambulance Service will be put into Special Measures, following concerns raised in a Care Quality Commission inspection.