If you catch the first train of the day (06.03) from Waterloo East, it'll get you into Canterbury West just before 08:00. There's a traffic free route which takes you out of Canterbury (with one short steep climb - which I usually walk) via Fordwich, and from then on, on tiny country lanes for ~18 miles, to Sandwich, during which you'll not encounter more than half a dozen cars at that time of day. Once in Sandwich there are plenty of places to grab a bite. Or if you have miles in your legs, turn left and pedal the seven or so on dedicated cycle lanes along the coast, past Pegwell Bay disused hoverport (which is worth visiting as it looks like something not of this earth) one short sharp climb out of Pegwell bay followed by a clifftop and parade ride, and you are in Ramsgate where you can breakfast like kings in Miles Bar (Highly recommended).
Ordinarily, after breakfast at Miles Bar, I would continue traffic free,all the way around the coast to Whitstable, from where I'd return by train - that would take your mileage to around 60 - which might be a bit much - having said which, once you are in Ramsgate there is no shortage of rail-bale opportunities between there and Whitstable.
If you decide to do this, let me know and I'll email you a GPS track - unless you are local, or you know it, you'll be lucky to find the snickets which lead you out of Canterbury traffic-free.