csales:
You can maintain 80% of max heart rate for many hours.
On a 8 hour, 155 mile ride with lots of hills, my average heart rate according to my Polar was 150 bpm. My max was 182 bpm, so I averaged over 82% of pr max for 8 hours.
At the time, I had fantastic endurance and good speed to boot. Just about every weekend, my training partner and I were doing at least 80-120 miles a day, with at least half of the session ridden as fast as possible, like we were in a 2 man break in the Tour.
We attacked every rider in our sights and dropped everyone who tried to sit on our wheels. Sometimes we would have to accelerate up to 26-30 mph for a several minutes to drop stronger riders to give you an idea of how hard we were pushing.
You need to do some long rides like 100+ miles with hills on a road bike to get ready for 62 miles off road.
If you don't have decent power output in the last hour(s) you will finish poorly and be miserable as you try to finish.
You can only get this long-term power by hammering big mileage rides.
Find a training partner who is motivated and push each other hard on the ride. Don't ride 100 miles slowly at 13-14 mph like many riders do. You need to hammer, recover, hammer, recover, etc.
Ride like you are in a 2 man break, smoothly swapping pulls at speed. When you get tired, chill out and ride easy, but push more when recovered. Ride wheelsuckers off your wheel, sprint for signs, attack other riders, etc. Imagine you and your partner are a freight train and you are both "delivering the goods."
Sometimes people freak when I say to ride 100+ miles. Many older riders in their 60 & 70's do 100+ mile rides every week, some do double centuries.
If you and your partner could average 17 mph, that's less than 6 hours of riding. When you think about it that way, 100 miles really isn't that big of a deal.
Good luck!!!