IMO, the key here is that Chaco has been building mileage and can complete an 85 mile ride. Which basically means he's removed the "unknown" factor from the ride: he can complete a 100 miles.
His daughter, OTOH, has never ridden, as far as we know, more than 90 minutes at a shot; perhaps the equivalent of 30-35 mile ride. All the other stuff about age, gender, weight training, general fitness levels, etc. is utterly irrelevant when we take these numbers into account. We know Chaco is capable of a 100 mile ride. We know only that his daughter is capable of a 30 mile ride. And that is the bottom line, IMO.