Fortunately, or unfortunately I live 1/4 mile into the red route, so know the route very well after using it for my training runs.
Red Route : first 2 miles are flat, then steep downhill for 1/4 mile then steep uphill for 1/4 mile, downhill for 1 mile past the 3 mile marker then flat all the way to Tower Bridge, up and over bridge then flat until you return back to Tower of London, slight downhill onto embankment, uphill for 200m under Blackfriars bridge then it's flat all the way to the finish.
Blue Route : first 2 miles are flat then very slight uphill for 1/2 mile, then it's downhill for next mile past 3 mile marker and join the red route.
Personally I'm pleased the cobbles have gone after blistering up on them at 21 miles last year, but each to their own, I really like hills, so figure that out!
Hope this helps & good luck.