The GU towpath surface in London isn't too bad, and these sections are much less busy than the Regent's Canal which is the one most people know.
That said, I wouldn't be entirely sure that the canal is the best choice for your route. The one you're crossing at Greenford is the GU Paddington Arm, while that at Hanwell is the GU Main Line. Going from one to the other by canal you have to head out to Bull's Bridge (Hayes), which isn't necessarily the most direct route.
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