Meet Straford International
Train to ebbs fleet 15 minutes ish
Ride 13 miles to Otford.
Then follow route below.
Quite possibly the nicest cycle to the coast from London there is.
You quickly reach the countryside south of Dulwich, & then it's just miles of lovely rolling South Downs cycling. The stretch on the Pilgrims Way Area Of Outstanding Natural Beauty is simply divine.
A couple of lung busting hills: Hollingbourne Hill being the worst by a margin.
Plenty of lunch options: The Kings Arms in Boxley has a big beer garden (kingsarms-boxley.co.uk), & the next couple of villages beyond this have good looking pub gardens too.
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train back to Whistable to Stratford