The Cycling Schedule 101

USA Cycling Coach Selene Yeager has this to say about training for distance.

"Distance riders often skip SPEED WORK because they  think they need volume, not intensity, to go long.  But riding fast improves your endurance by raising your lactate threshold, the point at which your muscles scream "Slow down!"  When you raise this ceiling, you can ride faster and farther before your body hits the brakes.  Once a week, aim to do four to six very hard or max efforts ranging from 30 seconds to two minutes on one ride; in between, spin easy for twice the length of the interval.  Do these on a challenging stretch of road, such as a hill or into a headwind.

Pro Triathlete Caroline Steffan - images from Timothy Carlson 

The other two rides in a three times a week schedule should be a LONG RIDE in Zone 1-2, which will recollect the familiar feeling of fatigue -- all too familiar in century races.  And a STEADY RIDE, where you aim for two to four longer efforts (15 to 30 minutes in length; 15 minutes easy pedaling in between) that increase your heart rate to 80-85 percent of your max heart rate.