Kim mentioned contact points. Seconded. It's hands, knees and bumpsydaisy. Hands: wear padded gloves, consider a second layer of tape on your bars to reduce road vibes. I get tingly fingers leading to dead hands and the extra tape and gloves keeps that at bay. Knees is code for position. I abandoned a 300 Audax because a mis-aligned cleat led to knee pain. Wholly avoidable if your position is tried and tested: saddle height, reach, bar height etc. Comfort, a relative term, is the key. Bumpsydaisy: a well worn-in saddle is worth its weight in gold.
The best advice I had (from Hellymedic) was to eat something every hour, even if it's only a boiled sweet or something to keep your fueling up. Most distance riders, and indeed racers, have experienced the bonk - hypoglycemic shock - at some point when you run out of energy in a dramatic and unpleasant way. The hammer falls and you have nothing left to give. Again, avoidable with feeds. Lot's of personal solutions ranging from banana sandwiches (yuk!) said to be an old pro fave, to energy bars and gels and everything in between. I like a slice or two of Soreen malt loaf, but that's just me. Rest for at least 30 mins and eat well but not to excess at your lunch stop. Keep your fluids topped up.
And my third key is spin don't grind. I try to keep to a steady-ish 75 revs/min pedal stroke. In my old club days we were supposed to train at 90 but I could never manage that. Either way a smooth gentle spin is a lot easier on the legs and knees - I'm getting old! - than grinding a bigger gear.
None of the above is an any way original, but it's common ground for longer rides, distilled through the experience of many over thousands of kms
