In reply to Professor Bunsen:
That's a long way!
I go from Uxbridge and use the Uxbridge Rd to Hanwell and then A4 to Chiswick.
I used to work in London Bridge and would cross at Hammersmith bridge as North of the river to central seems like the Tour de France peleton, which was fun on occasion, but i found south London is way more chilled out and less stress.
Google maps seems to favour the canal, I'd avoid it (I've been living on a boat on the canal for 9 yrs) as the towpath is narrow at many points, is gravel in many places, often has has pedestrians and it's quite unpleasant for them and you if you are trying to keep a pace. Not fair on either party really, which is why I stopped using the towpaths.
I'd personally get to Richmond, on the south and cross Kew bridge, (or get to the A4 before) go down Chiswick high rd to Hammersmith and go south again through Parsons green (past the Climbing Hangar, bouldering opportunity!) and stay south as far as you can until you have to cross the Thames again to central London.
It seems really long winded, but I did it 4/5 days a week for 2 yrs (46 miles a day) at around 90 mins each way.