More on pacing yourself for a century with a heart rate monitor:
Although elite athletes may sustain efforts corresponding to 92% of their maximum heart rate for one hour, after beginners have the strength and endurance, they ride at about 80% of their maximum heart rate. Since a century represents many hours of work, the level one can sustain will be considerably less. Elite racers finish a century in about 4 hours, averaging more than 80% of maximum heart rate. Beginners finish a century in more than 8 hours, averaging 65% to 75% of maximum heart rate.
— Baker, Altitude Climbing Endurance Training for Cyclists, 5.25 ed.