Within the 21 Day Cycle is a series of 3 day cycles. More than anything the best approach to each 3 day cycle is an approach, a mindset, towards each of the individual days.
At its core, the 3 day cycle is this:
Day 1 involves strength training and easy aerobic training.
Day 2 involves easy, slightly more demanding aerobic training.
Day 3 involves a longer and/or tougher aerobic training session.
Day 1
Of the three days, this day’s aerobic training should be easiest. If it needs to be a day off from aerobic training, then it should be a rest day from aerobic training, with only strength training.
Your primary focus on Day 1 is two things:
1) Get your strength training session done. Strength training can constitute whatever you individually need it to, though during base and race training I wouldn’t make it too tough. I do the Full Fourteen, which with efficient sets and 60 second rest breaks (longer as needed for transitions) takes me about 25 minutes.
You could can do a 20 minute strength routine, and rotate between different blocks of workouts. Since you’d strength train every three days, you could hit every muscle group in every single workout if desired.
2) This should be the easiest, shortest aerobic training of the next three days. I often take a rest day from running on these days. If marathon training, I probably do short easy recovery runs to build training volume. Generally, I often cross train on these days, though on weekends I’ll just strength train and rest completely from aerobic training on these days, since I typically train heavily during the workweek.
If you’re a workaholic, I’d recommend setting a Day 1 cap of 45 minutes on any aerobic training. This workout is setting the bar for Days 2 and 3, and if you set it too high you’re either not going to hit it or burn yourself out doing it. The idea of the 3 day cycle is to moderate your workload so you ebb and flow between challenging training and allowing for recovery.
Day 2
While this is also an ‘easy day’, Day 2’s aerobic training should be longer, more demanding than Day 1’s training. Three important points:
1) Keep this aerobic session to no more than 60-65 minutes. I saw that so if you’re cross training or running on a gym machine, it’s okay to get to 60 minutes and do the full 5 minute cooldown. Also, I don’t want anyone to freak out if e.g. they’re running outside and see their run has gone 1:00:23.
2) If you rested totally from aerobic training on Day 1, then any easy training on Day 2 will suffice. As a minimum, I recommend you train on Day 2 at least one minute longer than it took for you to strength train on Day 1. If I only strength trained 25 minutes on Day 1, I want to run or cross train for at least 26 minutes.
3) You however want to not train too much on Day 2, because Day 3’s training should be longer or tougher than Day 2’s. So you don’t want to set the bar so high that exceeding it on Day 3 is too difficult.
If you did aerobically train on Day 1, then you want Day 2’s training to last at least one minute longer than your aerobic training. So if I decided on Day 1 to do a super easy 20 minute run with my strength training, I’ll want to run or cross train no less than 21 minutes on Day 2.
If you want to do speedwork (tempo runs, track stuff, fartleks, a regular run with any sort of fast segments, etc.), or even a run on unfamiliar harder terrain like mountain trails, I’d pick either Day 2 or Day 3 for that.
But no matter what you do for Day 2 training, again keep it below 60-65 minutes. As I mentioned with Day 1, your mindset is to keep Day 1 short. Again, I’d recommend setting a Day 1 cap of 45 minutes on any aerobic training, so that this Day 2 workout builds on and proves tougher than Day 1.
Day 3
This is of course the longest of the three aerobic training days. On Day 3 you go longer, or you go harder, than Day 2.
When I originally wrote up the 21 Day Cycle, I recommended this aerobic workout be 60-90 minutes. That is generally true, and if you’re training for a race that will last longer than 90 minutes, you want your weekend Day 3 workouts to be sufficiently longer. Half marathoners should aim for closer to 2-3 hours. Marathons should build towards at least 2-3 hours, probably towards 4 hours if your goal time is that long or longer.
Otherwise, midweek, cap these workouts at 90 minutes. While you should aim for at least 60 minutes, if Days 1 and 2 were shorter, and anyting below 60 minutes would be longer than those two days, then you can go less than 60 minutes… e.g. Day 1 you rested from aerobic anything, Day 2 you went 30 minutes, Day 3 you could go as little as 30-35 minutes if desired or needed.
The general intention on Day 3 is to work on your aerobic endurance. But if you went 60 easy aerobic minutes on Day 2, you could on Day 3 work on speedwork. In this case, it’s okay to also do 60 minutes on Day 3, e.g. you ran easy for 60 minutes on Day 2, so on Day 3 you spent 60 minutes running repeats, or do a 60 minute tempo run, or an easy 60 minute run with a 15 minute fast finish, etc.
Otherwise, during midweek you can go up to 90 minutes on Day 3. This is where various research (that I won’t cite for now) shows your workout hits the peak of the bell curve on aerobic development. The only reason to go longer is to work on developing endurance for a longer race, and given the body’s recovery needs you generally want to avoid doing that more than once every 6-8 days. The sweet spot for aerobic fitness development in an easy workout is 60-90 minutes.
THE WEEKEND PIVOT
My original 21 Day Cycle recommends two long workouts within 6 days of each other, followed by two weeks off from any workouts longer than 90 minutes.
While this allows ample recovery from the long workouts, some may find this doesn’t allow them to sufficiently build up their long run in marathon training.
It also reduces one’s margin for error in marathon training, that if they have to miss one of their scheduled long runs, they’re now facing at least 3 weeks between long runs, which could lead to a loss or setback in fitness.
So maybe you want to schedule a long run every week, and want to schedule one during that odd 2nd weekend where Day 1 falls on a Saturday and Day 2 falls on a Sunday, leaving the longer Day 3 on a Monday. (And yes, the occasional Monday holiday makes a long run work there, but 95% of the time it doesn’t.)
If you want to run long every weekend, I propose an alternate 3 day cycle for those odd weekends:
Day 1: You strength train, and either take off from aerobic training or do an easy aerobic workout, as usual.
Day 2: Do your long run here.
Day 3: Either take this day completely off, or do the same sort of easy aerobic workout you would do on day 1 (but no more than 45 minutes).
Then the following day you go back to Day 1 as before and resume the normal cycle.
If your long run isn’t any longer than 2 hours, I would say this is not at all necessary, that you could follow the original 21 Day Cycle as usual. This Pivot is largely for marathoners and similar long distance athletes who find the scheduled weekend off from long training more concerning than rewarding, or otherwise feel they must be able to train long every weekend.
And of course, if you do need an easy weekend, you can always go back to the original 2nd weekend schedule as needed.
YOU CAN DO OTHER STUFF
I often cross train each morning throughout the week regardless of what other running or strength training I have scheduled each given day or week. I’ll do super easy, zone 1, 50-60% of max heart rate “wake me up” cross training a lot of mornings or afternoons on the spin bike or similar. I also take a lot of walks during breaks at work.
You will want to be careful not to overdo any of this, especially as you build serious training volume, but if comfortable for you it’s totally fine.
I often take it easy on the weekends. If I don’t have a strength session or long run on a given day, I’ll often take a total rest day.
IF LIFE INTERVENES, IT’S OKAY TO BAIL
You wake up on your Day 3 and you feel sick as a dog, or something hurts more than it should, etc? Just take the day off. Take a whole 3 day cycle off if you want.
None of this is legally binding! The cycle does make it easy to get back in the swing of things should you need to take a day off or an extended break. Obviously, if you need a long break during a trianing block, you still need to reconsider your race goal as usual. But live your life and make adjustments as needed.