The biggest question to answer is what your biggest limiter is. Is it the speed to ride at 21 mph at all, is it the aerobic endurance to ride at speed for 5 hours, or is it the postural endurance to sit in that position for 5 hours? For most people, it's a combination of all three.

There are two major ingredients for training at a specific speed (or power): pull and push. Pull means riding above that speed for short periods of time (usually intervals). Push means riding below that speed for long periods of time (usually as sustained intensity). With the kind of durations you are looking at, I'd probably look at something like 80% push, 20% pull.

The first rule of riding at 21 mph is to ride at 21 mph. If that's your goal for 5 hours, then it's a good number to use as a reference point.

What does that mean? It means that 80% of your training days are focused on riding as close to 21 mph as you can sustain for the time you have. If you have 2-hour ride, that might be 60 to 90 minutes at 19.5 mph. If you only have an hour ride, that might be 45 minutes right at 21 mph.

The remaining 20% of your rides have intervals that are above 21 mph. It might be intervals of 8 minutes at 22.5 mph, 15 minutes at 22 mph, or 30 minutes at 21.5 mph. It all depends on how long the interval is and how many you can fit into your day.

For centuries, intervals of less than 2 minutes won't get you much. It's not to say they won't help at all; you can just get much more benefit from other workouts.

Weight lifting won't directly improve your riding. It might remove some muscle imbalances and improve your fitness in general, but it won't make you a faster rider, especially over longer distances. Cycling at 18 to 20 mph at 85 rpm requires pushing a load of 55 pounds per leg per stroke. Unless you are severely lacking in leg strength, that is most likely not going to be your limiting factor.

core work and plyometrics!!!!!!

For a track sprinter or a crit rider, I'd agree. Not for someone looking to do centuries.

The first rule of riding at 21 mph is to ride at 21 mph. If that's your goal for 5 hours, then it's a good number to use as a reference point.

What does that mean? It means that 80% of your training days are focused on riding as close to 21 mph as you can sustain for the time you have. If you have 2-hour ride, that might be 60 to 90 minutes at 19.5 mph. If you only have an hour ride, that might be 45 minutes right at 21 mph.

The remaining 20% of your rides have intervals that are above 21 mph. It might be intervals of 8 minutes at 22.5 mph, 15 minutes at 22 mph, or 30 minutes at 21.5 mph. It all depends on how long the interval is and how many you can fit into your day.

For centuries, intervals of less than 2 minutes won't get you much. It's not to say they won't help at all; you can just get much more benefit from other workouts.

This is a good read. Appreciate it. I did something similar while preparing for a couple 200km rides (~125miles) last year. On long rides I focus on keeping a pace which I can sustain for 4/5 hours (a speed slightly lower than my "average"). Apart from endurance training I was getting myself able to be on the saddle for hours. These are twice a week several weeks before the event (lazy me..)

The other rides are often with a group most are faster than me, I got the intervals training there :grumble: . Also some hill repeats (just baby climbs nothing like yours!)