nearly everyone focuses on nutrition but an overlooked aspect is saddle time, foot and body feel. if your body/feet/undercarriage is/are sore as hell after 4 hours and 60+ miles in,
you'll only be thinking about that. get plenty of prior saddle time and loosen the shoes a little. don't forget to stand for 15-30 seconds every 5 miles or so to relieve pressure; keeps
certain muscles fresh and to give certain muscles a little break. if on a descent, look to unweight the saddle as much as possible and ease back on when you've gotta start pedalling again.
try to give equal time to the contact points of the feet, seat and hands. the seat usually takes the brunt of affairs.