I have to ride Lewisham to Soho but find it's not long enough and too flat.
For about a 14 mile ride in I go up to Ladywell, Forest Hill, Crystal Palace, College Road down to Dulwich and then Herne Hill, Brixton, Oval etc.
That way you get a nice brisk climb and it's fairly uncluttered as open road and buslanes most of the way.
For a 26 miler you can go out to Grove Park, Bickley, Keston West Wickham, Elmers end then up to Crystal Palace and in from there. A few good climbs. Takes about an hour and a half.
Good luck.