From Dartford I would go to Bexley village (where the station is) then Blackfen, Eltham, Lee Green. That route is bypassed by the A2 throughout so it's not that busy even in rush hours.
From Lee Green you can either go up to Greenwich Park and pick up Q1, or go into Lewisham and pick up Q1 in Deptford, or even the Old Kent Road isn't too bad.
The Yorkshire Grey roundabout is busy but not too fast - if you're confident it should be OK.
If you're not wedded to the A2 cyclepath, then from Rochester I would branch off through Cobham, Longfield, Wilmington, Bexley then as above. There are no ridiculous hills but the A2 route is probably flatter than this.