I did 40 miles down Thames Path today from the source to Newbridge. Given you're at that end I would steer well clear, certainly if you're thinking of doing it in the next month or so. Of the first ~13 miles, 2-3 miles were underwater and not runnable, over the knee in places. Further down water was intermittent but replaced by mud which again wasn't runnable, maybe with super spikey shoes but I'm not convinced that would do it.
[Edit: just checked a map and this is well short of 100m, you could extend TP down to Slough or go further out on the RW loop, just means a bit more map reading]
If you can get to Oxford then you could make a route using the Thames path and Ridgeway. Thames path down to Goring is about 25 miles, from there you can pick up the Ridgeway with about 40 miles either direction. I'm sure looking at a map you could find a decent loop ending up in back in Goring and then finish off with Thames Path to Reading (good trains). For the Ridgeway bit you could run to the White Horse and then pick up the Lambourn Valley Way which I think takes you all the way to Newbury. If you can get a link NewburyGoring then it should all be easy to follow.
Bing Maps let you see 50k OS maps so you can scope it there.
Edited: 16/07/2012 at 21:53