Greg took a training session at the Eldon Square shopping centre as part of the launch event.
He won gold at London 2012 long jump and says now it's his turn to inspire others.
More top news
The stakes are high for the Labour leader, writes Chris Ship - will another general election be on the cards in the coming months?
Kerrie Gosney brings you Saturday night's forecast
Showers becoming heavy and thundery Saturday afternoon. Drier for a large part of Sunday