Tony, the trains, tubes and DLR get quite busy so I would leave nice and early, you don't want a mad panic getting to the start. You'll also want to settle down, use the loo's (the queues get quite big) and drop kit back off. The route is mostly flat altho there is a copule of sneaky inclines, one at mile 3 where the blue and red starts merge and one right near the end, around mile 25 when you come out of the underpass at Blackfriars. They are quite gentle tho and I wouldn't worry about them too much,