Giles pretty flat generally. A slight incline going out towards Brighton marina approx mile 4 I think. Hove to Portslade is my worst part approx mile 15-18 but there is very good support there. Most people don't like the shoreham power station part at mile 20-23 ish as you get tired by then and not much support and not attractive.
Saying that overall it's a great marathon with a great atmosphere. Wind is generally from the west so behind you on the home stretch.