Anchor Road, Canons Road, Canons Way, Lower College Street, Bridge Valley Road, the Portway, some roads around the Cumberland Basin System and others will be closed early morning until mid-afternoon.
The race starts at 09:30 from Anchor Road, Harbourside.
More top news
The first two days of the "proper" General Election campaign have seen visits to Wiltshire by David Cameron and Shadow Chancellor Ed Balls.
Statistics show month of March to be sunnier than average despite a very windy and wintry period of weather in last 24 hours.
As we leave winter behind and re-discover daffodils and longer days, we showcase your shots of West Country spring.