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Look at the Down Tow Up Flow half marathon from Windsor to Marlow (or the other way around). It's probably about 95% towpath with only a couple of short road/pavement sections. Fairly well organised, and a nice route along the Thames (so pretty flat).
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If you're willing to do a super-hilly one, the only off-road half I've done is the Midsummer Munro, which takes you 3,000ft of elevation (hence the name) over the North Downs, including those lovely steps by Box Hill... twice. Not sure what your knees will make of it but your quads won't thank you. Classic race!
I'm thinking about doing a half marathon next year, is there any in particular that people would recommend? Preferably one that doesn't cost the earth, is somewhere around London, and (this might be a long ask) is mostly off road (pavement pounding is awful on my knees).