I do some lumpy rides at the crystal palace area... but must admit i am not fan of any hilly rides.
Well, if you're local to Crystal Palace and want some quiet lanes then rather than go out by train you could head straight through Elmer's End to the West Wickham/Hayes area. From here you have a nice area containing many quiet lanes (quite busy with cyclists on the weekend) bordered on the South by the Pilgrim's Way (which is at the bottom of the steep slope of the Downs) from roughly Woldingham (W) to Halstead (E). Avoid the main road through Biggin Hill that cuts the area in two as that's not a very nice road to ride on.
A nice ride would be to follow Layham's Rd/Skid Hill La/Chelsham Ct Rd/Beech Farm Rd (it's all one road really) from West Wickham all the way up to Botley Hill (where there is a cafe I think, but I've never been in it) Then down Clarks Lane and pick up Chestnut Avenue (unpaved in parts) to Hawley's Corner and head back via Downe. Not quite 50 miles, but easily extended. And not quite flat either
That's not what you asked for, and there are some excellent suggestions above that
are what you asked for, but I'm just saying - don't ignore what's on your doorstep.