You don't want to do that.
Your "easy" efforts are too hard, recruiting fast twitch fibers so you don't train your slow twitch fibers.
Your "hard" efforts are too easy so you don't train your fast twitch fibers.
You go hard enough to reduce appetite suppressing peptide YY production, but not hard enough to reduce appetite stimulating ghrelin. You shift your energy substrate utilization towards carbs, deplete your glycogen stores, and hunger to replace them. Runners call the result "runger" where you can more than eat back the calories you burned.
https://www.quora.com/How-should-I-t.../Drew-Eckhardt
Total power is the sum of aerobic and anaerobic components. Target both with aerobic requiring more training time and intensities between the two having limited benefit for speed. Learn to pedal faster. Lose weight if the course has hills.
Figure out your aerobic (AeT, VT1) and anaerobic (AnT, Critical Power, FTP, LT4, LTHR, VT2) thresholds.
One day a week, ride 7–10 minute intervals as hard as possible, stopping when you can’t exceed your anaerobic threshold. Stephen Seiler’s research into polarized training with Olympic and other endurance athletes is very relevant.
Ride below your aerobic threshold four days a week, including a long ride at least double your normal distance. One hour short rides are OK but two are better.
Go for a fast long ride one day a week for experience pacing and to see how you’re doing.
I like a FTP test every 3–4 week mesocycle to quantify gains.
Otherwise riding between the two thresholds won’t do much for speed once you have a bit of fitness - it engages your fast twitch fibers and glycolytic energy system so you’re no longer stressing your aerobic fitness to force improvement, is not hard enough to stress your anaerobic fitness, and adds more fatigue than riding with lower effort.
Take an easy week out of every 3 or 4 without the hard rides and lower volume to recover. Growing older makes needing a rest week in three more likely. Add 10% on your non rest weeks.
Change things like rest week volume and frequency when you’re having issues.
Track training stress to insure you’re not adding too much each week and have easy enough rest weeks to allow adaptation.
Your aerobic threshold is where breathing becomes rhythmic, conversation doesn't flow, and lactate/hydrogen ions start to accumulate. It's an intensity you could sustain for 3-5 hours with an even split between halves.
Mark Allen set his 2:40 Ironman marathon split record which stood for twenty-five years after training below his aerobic threshold, initially dropping his pace to 8:15 miles with performance improving over a year to 5:20 at the same 155 bpm heart rate.
Not coincidentally, the aerobic threshold heart rate is often close to that predicted by Phil Maffetone’s formula. He coached Mark Allen.
Your anaerobic threshold occurs around your average heart rate over the last 20 minutes of a 30 minute all-out effort. In theory you could do that for one hour although that's very unpleasant -
Eddy Merckx, the greatest cyclist ever, said it was one of the hardest things he'd done on a bike.
It's very, very hard.
I couldn't walk for a few days after I did it.
That's how hard it is.
It’s approximately your Critical Power, or Functional Threshold Power where 95% of 20 minute power is an estimation varying in accuracy because people’s anaerobic reserve (Anaerobic Work Constant, w’) differs.
Training stress is approximately proportional to the square of exertion, with quadruple the time possible at half the intensity. It can be quantified on long term (fitness, Chronic Training Load, Long Term Stress) and short term (freshness, Acute Training Load, Short Term Stress) bases as exponentially weighted stress averages with typical 42 and 7 day periods. There is a delta subtracting short term from long term (Stress Balance, Training Stress Balance) reflecting how much of your fitness you’re using. You need negative balance for endurance increases, and to approach zero for recovery allowing training adaptations to take place.
For sustainable increases and sufficient recovery, track it with performance management software like Golden Cheetah (which runs on Windows, Mac, and Linux) or Training Peaks. Prefer a power based system like Coggan’s using Training Stress Score or Skiba’s BikeScore, although Banister’s TRaining IMPulse points work.
Use a power meter to quantify anaerobic performance and monitor how close you are to that threshold. Heart rate isn’t useful for hard workouts because it’s a lagging indicator. Too much exertion will limit duration by exhausting your muscle glycogen in less time (it can’t move between fibers) and causing fatigue proportional to the square of exertion.
Use heart rate for pacing long efforts because it’s fairly constant at your aerobic threshold although power will change significantly with training.
Learn to pedal faster because higher cadences are less fatiguing. The hour record is usually set at over 100 RPM for this reason. A cadence field on your bike computer is a useful reminder when you’ve been off your bike for a while and your legs are sluggish.
If there are hills on those 23 miles, eat less and exercise more. You’re too fat to climb quickly if you can’t see your abs.
With a decent aerobic base, you can probably manage up to 4 hours riding without food which is a half a pound of fat. Eating 1/3 of your energy on long rides can be sufficient, with a 200 mile ride using up a pound. Accounting for metabolic efficiency, assume 1 kilojoule out is 1 kcal in.