Just been doing a bit of planning for next week. I'm tempted to trim the distance on my DIY (A&S-ish) 400km by taking the old A120 (B1256) to Colchester. Some bits on the proper 120, but not too busy at early o'clock (06:00 start from Dunmow) - I've done this a few times to Manningtree and it's much shorter (by about 12km in my reckoning), flatter and less laney/fiddly than the pretty route through Bures to the Vale of Dedham. Some lorries about, but the traffic doesn't get too bad until the A12 crossing, then it's cyclepaths (ok, a bit stop/start) alongside the Avenue of Remembrance and Cowdray Avenue, then out of town onto the old Harwich Road to Ardleigh and Manningtree.
All this is hypothetical if you're on a mandatory route by gps, but I'll be collecting traditional p-o-p.
For my 300km I'm doing the opposite, for the sake of riding some new areas in Norfolk, adding only about 6km, I think and staying off the roads we'd normally be riding in the dead of night.