According to the official web page, the race will be centred around the Dome, sorry I’ll say that again, the O2.
There are lots of places where you can get free / public access to the riverside in that area, and should have an excellent view of the action.
On the south bank of the river:
There is a public footpath on the riverbank all the way from Greenwich (Cutty Sark) to the Thames Barrier, passing right in front of the dome/O2. Nearest stations are Cutty Sark (DLR), North Greenwich (Jubilee) or Westcombe Park (Network Rail). Some of this footpath goes through surprisingly industrial areas, and the smell from the molasses refinery can be a bit iffy.
On the north bank of the river.
• Access to the river is more limited, but there are some places where you can get to the river, and get a good view.
• On the Isle of Dogs, there is public access by the entrance the West India Docks. Nearest station South Quay (DLR)
• There is public access in front of the Jamestown Quay development . Nearest station East India (DLR)
• There is a public footpath (which has the advantage of a little extra height because you’ll be standing up on a bridge) alongside the Lower Lea Crossing. Nearest station East India (DLR)
• There is a small patch of riverside access in Lyle Park. Nearest station West Silvertown (DLR)