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In theory it is possible to view at multiple points and I have had success at this, primarily watching people running sub 3 hours.
This year, though, my wife found it very very difficult to get away from Westferry and struggled to meet people around there due to the density of the crowds. I also ended up on the embankment around 4.5 hours in and it was uncomfortably busy. So maybe do a spot at the beginning and then a spot at the end.
Public transport all the way. Not surprisingly it gets busy on the DLR and relevant tube lines but it's very doable.
Suggested itinerary (I've done something like this a few times...)
Start: Creek Road, between Greenwich and Deptford (mile 7) NOT middle of Greenwich which gets really busy. South side of road so you can access Greenwich DLR station. Then DLR to Westferry. Here you can see runners at 15 miles, as they head from Narrow Street to the Isle of Dogs. Then cross the road at your leisure to East India Dock Road (just over 20 miles) before getting back on DLR to Tower Gateway, and either take your chances trying to get another view around 23 miles, or tube it to Embankment or Westminster for the final mile or two, where it's even busier, but then at least you're not far away from meeting at the end. (Walk down Whitehall and through Horse Guards where the meeting area is.)
All sorts of other options for multiple stop-offs using the DLR, as Docklands is much better for viewing than some other places, although Canary Wharf itself also gets very busy.