This was my very first 600 back in 1984.
Some of us started from a tent just north of Shifnal, then Stow-on -the-Wold, Bicester and down to Beaconsfield. Then back to Bicester, then a workmans hut near Martin Hussingtree, Shifnal, Prees Heath then up into Cheshire to the organisers house. Retrace to Shifnal to finish.
The roads were quite busy even then.
Next time I did it was 1992 and the start was in Kidderminster. Route was basicaly down to Marlow then back to Kidderminster. Up to Knuttsford services and back to Kidderminster.
Traffic was much heavier by then.
Not too hilly for a 600 and not that much scope to go off-route either.