Returning for the twelfth time, the event will see super-strong competitors complete a 140-mile triathlon starting at 6.30am on Sunday.
Residents planning to head out for the day are also being advised to check for road closures which will be causing disruption throughout the day.
The start line is at Pennington Flash in Leigh, where athletes will take part in a 2.4 mile swim in the open water.
They will then endure a 112 mile bike ride, going through Adlington, Heath Charnock, Croston, Lostock, Leyland, Buckshaw Village, Euxton, Whittle-le-Woods, Brindle, and Wheelton.
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The event will then be completed with a 26 mile marathon in Bolton.
Routes will be closed at different points during the day - spanning from 6.30am until midnight
Among the roads being closed for the event are the A6 Preston Rd, Highfield Road, Dawson Lane, Runshaw Lane and Langdale Road.
Full closures also will be in place on Copthurst Lane, Birchin Lane, Denham Lane, Water St, Sandy Lane, Coppull Moor Lane and Jolly Tar Lane.
In Lostock, Regent Road and Rumworth Road will be closed completely for crossing from 6am to 5.30pm as the bike ride will travel in both directions along this route.
The A49 Preston Road and the A674 Chorley Road are allowing local access but delays are expected on these roads.
Ironman is also warning people about delays in other areas of the region where the cycle route does not pass through.
For information on road closures, diversions and the event should visit the Ironman website at eu.ironman.com.