I do a similar distance, I built up to it though. I used to do 5 days a week but was a bit of a tired and grumpy dad at the weekend so cut it down to 3 days, sometimes 0-2 depending work commitments and gales.
Sounds like you dived straight into the deepend, why not just commit to 3 days a week and aim for Sevenoaks, then Chelsfield when your naturally ready.
Do you have to stand on the train from Chelsfield, that would do my head in after a 15+ mile ride, asking for cramps too, you must be a sweaty mess :wink: