It's a bit longer but it is very well signposted so would save looking at a map and it tends to take the more scenic routes away from main roads. If it's too long then you could always train the Norwich-Cromer, or London-Colchester/Ipswich bits at either end
Could follow National Cycle Route 1 for most of the way
http://www.sustrans.org.uk/ncn/map/route/route-1
It's a bit longer but it is very well signposted so would save looking at a map and it tends to take the more scenic routes away from main roads. If it's too long then you could always train the Norwich-Cromer, or London-Colchester/Ipswich bits at either end