I've done plenty of 200s and am looking forward to my first 300 next weekend, and feel ready for it (famous last words).
In my experience a 200 is 'just' a long day in the saddle but going longer takes in new territory. I'd welcome what your most important tip is for not only a 300 but going through to an SR, and if lucky enough to qualify and get a place, PBP.
It might be mental approach, kit, control and feeding strategy, or how to approach night riding when your body or mind says you should be in bed or in the pub.
So far the most important thing I've learnt through experience is that there often comes a moment on rides when you wander why you turned up, but that moment passes and the happy days return; that knowledge gets me through the dark spots.
What I have in enthusiasm I lack in experience, so first hand tips gratefully received. Hopefully a few of this parish may agree on a few key points which will help me greatly.
Thanks.