I rode the route today. Well, I rode my own version of it. Obviously I couldn't use the A12 so I joined and left the route at the Green Man Interchange which is where the ride leaves the A12. I also avoided the A414 East of Epping and West of Chelmsford .
All in all I think I rode about 55% of the actual route.
I's lacking in significant hills but it does undulate so there are some leg sapping inclines - but nothing steep or long, but with tired legs they could be dispiriting. But the payback is some long gentle downhills which are great.
It's not the flattest century I've ever done. The Fridays ride from York to Hull via the super-flat vale of York (with extension to Driffield to make up the ton) had significantly less climbing.
It's obviously designed to be able to handle 10s of 1000s of riders. The roads are all significantly wide, which doesn't make for a great solo ride as this means they are also fairly busy. Not horribly busy, but not the best choice for riding on your own.
I had to hack my own way through central London to Woodford and then back on the return leg. That was not enjoyable in the least, especially on the return leg. Very stressful. I guess that's what closed roads are for!