Zones

Only time I ever went Z5 was during end of races (sprints & run races), never a plan during training.  Run training I was probably 90/95% Z2, drifted into Z3 during end of long runs and rides. Periodic Z4 if I did run intervals, usually the 4-6 wks before an important sprint or run race.  Biking, if I did the group ride thing they would always have one or two points where everyone would play 'King of the sprint' starting about a mile out that were punishing, maybe exceeded bike Z5 threshold for a very short duration.  Otherwise most of my then bike training was on a trainer using virtual power so not sure what the HR's were.

My plan for about 12 years after college purely as a runner was to do speed work every Tuesday (20 minutes in zone 4), Tempo Run every Thrursday (30 minutes Zone 3), long run on Saturday (2 hours in zone 1) and then to do the rest of my running in zone 2 (3 hours).  

So that came out to about the following:

Zone 1:  50-55%

Zone 2: 30-35%

Zone 3: 5-10%

zone 4: 5-10%

I never looked at the break down in percentages (until just about 2 minutes ago) and didn't even know what my heart rate zones were until about 18 months ago.   I just knew that on Tuesday I was going to lay in all on the line.  I knew I was pushing hard enough when my lungs were burning so much that I could taste the blood in my saliva.  On Thursday I knew it was going to be a faster run but really fun.  On Saturday I knew that I was going to be going painfully slow but that I was going to be on my feet for a really long time.  The rest of the days I was just doing that I felt I needed to do to be ready for my next key run.  If I was dead legged I would slow things down.  If I was feeling well rested I would go faster (but rein in the horses so I could save my legs for the key runs).  

NOTE: I have been doing the "zones" all month for my running.  my 30 minute test was a 162 BPM thresh hold HR and my thresh hold pace was 6:11 min/mi.  This morning I did 9.25 miles and if you calculated my zones by HR my L1 zone is everything under 136 BPM which put 54 minutes of my run in the L1 zone. If you calculate my zone by pace my L1 zone is everything over 8:00 mins/mi which only put 17 minutes of my run in the L1 zone (basically just the warm up and cool down).  So...the way you test and define your zones can make a huge different.  The 80% slow 20% fast is a rule I have come across.  My miles look like they pretty much met that rule of thumb.  I also have heard that the two biggest problems that amateur athletes have is that they 1) don't do their fast workouts fast enough and 2) they don't do their slow work outs slow enough.  So make sure you are really work the upper and lower end of the spectrum so you get the full range covered.

First thing before we talk about ratios - the biggest mistake most age-group triathletes make is they don't go easy enough on easy days and they don't go hard enough on hard days.  That's probably the single most important thing.  If you aren't training at the right intensity, it doesn't matter what ratios you are using.  The natural inclination is on easy days to push just a little bit harder, because surely a little bit harder will be better.  Unfortunately it's not.  You push just a little bit harder and you are going too hard to train the aerobic energy pathway, but not hard enough to train the anaerobic energy pathways.  When you go EASY you are training the aerobic energy pathway.  When you go HARD you are training the anaerobic energy pathways.  When you go "just a little bit harder" you are doing a lot of work, but aren't getting much return for that work.

Now that we have that out of the way, when you start talking about the ratio of easy/hard, you need to understand there is a difference between cycling and running.  Running is a high impact - the faster you run, the greater the impact forces.  Cycling on the other hand is considered low impact.  I know swimming wasn't part of the original question but swimming is no impact.  Your body can tolerate FAR more intensity while cycling or swimming then it can running.

In general, a mix of approximately 80% easy to 20% hard for running works reasonably well.  Easy being Z1-2, with hard being low Z4.  If your 'A' race is a sprint or Olympic then maybe some Z4-Z5 work from 6-8 weeks before the race until the beginning of the taper.  There's some debate over the value of tempo and/or speed work for long course triathlon.  On the one hand, you can already run faster then you could run for 3+ hours and shifting your lactate curve right with LSD runs will likely provide more improvement then speed work could.  On the other hand, speed work will make you run faster.  The question is will that speed work make you faster at a sub-maximal effort that would be equivalent to a long-course triathlon run effort.

Cycling intensity likely will vary depending upon where you live and what phase you are in.  In cold weather climates where you are riding indoors during the winter, the ratio might be something like 50% Z4-5, 50% Z2-3, when it gets further into the season and you are doing a long-ride outdoors, the ratio would likely go back closer to that 80:20 Easy:Hard ratio.  In warmer climates where you can ride outdoors all year the ratio would depend upon whether you choose to do a winter bike focus (50% Z4-5, 50% Z2-3) or stay with the 80:20 Easy:Hard ratio all year.

Hope my humble two-cents helps.

My thoughts echo Scott's almost exactly.  The only thing I can add is one of the reasons to vary the ratio at different times of the year is because your training objectives vary throughout the year.  For example, I have some athletes whose goal this time of year is dropping fat and improving their body composition. (I avoid having athletes trying to lose weight during bigger volume race specific periods, because it can undermine their fitness gains.)  Their volume is already lower because it's the off season, so when we mix in a fair amount of HIIT into their strength program for the purposes of EPOC and getting more bang out of their limited training hours, it ends up being a much higher percentage of their weekly volume than any other time of the year.  At the other end of the spectrum, for long course athletes during race specific training, I don't have them do any z5 work, so there is a significant difference in the ratios from the high point to the low point.

Begin with the end goal in mind and design your training based on it.