It is true that rides over 5 hours won't improve endurance performance. However long hard rides aren't really about performance. A century doesn't count as a long ride. You're still in the "I can do that on any given day" distance range. On a long ride the issues shift to hydration, pacing, fueling, rest stop timing, and uh, pain tolerance. I'd never experienced ride-stopping pain until my first hilly 300. No, I didn't stop. Small pacing and nutrition mistakes, hardly noticeable on a century, become big deals. So that's why it's a very good idea to do a couple really long rides before your event. Plus you'll gain a lot of confidence. No need to ride an organized event. Map out your own routes with equivalent gains. Be sure to put a couple nasty long steep climbs in there, where you run out of gears. Nothing like a nice 18% climb when you're a 150 miles into it. You['ll want resupply points every 50 miles or so.
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