Heading out from London there are masses of decent days out which can easily be linked into two or three day expeditions. Just build up the distances gradually so you don't turn it into an outdoor treadmill. http://www.cyclingfromguildford.co.uk/
was a website I dipped into to widen my cycling horizons- they are good for a wide area of the south east - not sure whether the guy writing it posts on here but its an excellent site and gives lots of advice on route surfaces and rail links etc.
You could do worse than head out along the Thames towpath and then drop onto the Basingstoke canal from Woking to Fleet where you can join the KAW in either direction to Reading or Farnham.
North Kent also makes for extended easy off road coastal cycling most of the way to Margate.
If you want more mud and bumpy bits linking the Downslink with sections of the South Downs Way and then getting a train back from Brighton, Eastbourne or Havant would be fun.
North of London there are lots of canals and old railways asking to be connected with the Ridgeway The Icknield Way and bits of the Peddars Way all the way to Norfolk if you fancy! Use cycle.travel which is the best route planning mapping for cycling IMHO.