Is this the Portsmouth one?
I f so it is one my wife and I helped set up for the local Councils before it was taken over by BHF
Where are you starting from?
The route from the Gosport ferry to Hayling Island is flat, as is the Hayling Billy to Havant. Then ypu have Portsdown hill which is a long slog, but excelent downhill to Fareham. Fareham to the Ferry is also fairly flat once you reach the coast.
There are shortcuts!
You can train from Havant to Portchester or Fareham and rejoin, or from Havant link across to Fareham at a low level and miss the big hill.