Originally Posted by johngwheeler

Aha! I may had a Eureka moment. I tried the suggestion of moving the saddle back (15mm), and also titling it upwards slightly, and this seems much better. No more sliding forward.

I think part of the problem was using a saddle (Fizik Antares) that has a flat front section, but a ramped rear section. This saddle is only comfortable me if I sit at the widest section in the rear, which is only comfortable with a margin of +/- 1cm along the saddle's length. Because of the "ramp", I was sliding down it to the thinner portion that caused discomfort.

I also think that getting my weight more to the rear, and applying a greater rearward push with each pedal stroke (because my legs are pushing from further back) helps keep me in this position.

First Century ride tomorrow, so let's see how this goes! Fingers crossed it's not a short distance cure only....