I like one workout per week to be structured intervals and a second one to be unstructured. The majority of athletes can handle at least one interval day with higher intensities during the week and a long ride with intensity on the weekend to form the cornerstones of their training plan.
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Add Hills to the Intervals
After lactate threshold fitness has been built on mostly flat terrain for some three to eight weeks, it's time to add hills. Do the workouts described in the lactate threshold section on hills or do repeats up a single hill. If you don't have hills where you live, simulate them by using a harder gear or pedal into a headwind for the work bout.
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Back-to-back Tough Days
After progressively building endurance and the ability to ride for longer periods of time at higher intensities (higher power outputs), do difficult workouts on back-to-back days.
For example, four to eight weeks before the event, do structured intervals with hills on Saturday for a workout in the 2-3 hour range. The next day, do a ride in the 3-4 hour range where you ride every hill as fast as you can, at the highest intensity you can muster. Do not worry about watching your heart rate on these, just go.
If you don't have hills, simulate them as previously mentioned. Another option is to make this second ride a fast group ride. Usually, other riders can help you ride faster than you would on your own.
More advanced riders can include a weekday ride with intervals above threshold, in addition to the back-to-back weekend rides. One example is to warm-up, then do 3 to 5 x 3 minutes on a hill that gets your heart rate into Zone 5b. Hold it there to the end of the interval time. Recover with easy coasting downhill and spinning for 3 minutes. This is a 1-to-1 work to rest ratio.
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Be sure to peel back the training volume and keep some intensity in the week or two before your century ride. While you're busy resting, make certain your bike is in great working condition.
Plan to Finish Strong
On event day, ride the first 50 miles at a lower heart rate, power level or perceived exertion level than the second half of the event. This means you will be riding the first 50 at a pace that feels too slow to you. That's okay. For example, if you use heart rate to guide your training, ride the first 50 miles at heart rate Zones 1 to 3. Bring it home strong at heart rate Zones 3 to 5a.
At the half-way mark, ride the last 50 miles at the fastest speed you can manage. Use riders in the distance as goals or imaginary competitors. Ride fast to see if you can catch them—on a climb.
More: 12 Tips for Your Next Century