TID….Training Intensity distribution.
In simple terms, this is breaking down your training into three different buckets: Moderate, heavy, and severe.
In the moderate bucket, you can sustain training for multiple hours at a time. In the heavy bucket, an hour or maybe slightly more, and finally, less than 30minutes in the severe domain.
In endurance terms, the first transition may be roughly equivalent to the marathon pace and the second roughly 10km pace. These bands could transition over quite nicely into CrossFit or any training modality for that matter. Long steady chippers vs. 12-15min workouts vs. Fran.
The optimal TID is going to be dependent on many factors and is entirely individual to you.
Genetics, training history, time commitments, stress, recovery, nutrition, sleep, menstrual cycles, etc., will all play a role. What we can identify, though, are specific patterns that emerge when we look at successful athletes.
Most of us do not have the luxury of time to train like or be professionals in our sports. Elite runners will most likely have two sessions on most days of the week. For example, they won't run for 90mins with intervals in the middle. More likely, there is a dedicated intervals session with a dedicated easy session. Their time is split into clearly focussed buckets. This is something we can take away as amateurs or new athletes.
You will not become a great ultra-distance runner by running 50km per week. You might get away with it, but at some point, you will need to run more if you are going to be a good ultra-runner.
If you are running 100km+ weeks, you will not last long if most of those runs are at a 10km pace – it's just not viable. Most of this has to be at an easy or moderate effort to run significant mileage for longevity.
Running low volume is not enough and running fast and long is not sustainable. Again, this same principle can apply to CrossFit or strength training.
Most people should look at the total time or distance they are spending at each intensity and not simply the number of sessions as an intensity. Track Tuesday, for example, has a warm-up, drills, recovery time and a small portion of that hour is the intervals themselves. The same applies to a mainline class: The warm-up, the strength or skill, and then the workout.
As a general rule, 85-90% of your training TIME should be spent at moderate intensity, most of the rest in the heavy domain, and the final % is severe.
With this simple split, you will allow yourself the time to recover and adapt, avoid injury and overtraining risk, and should continue to progress in your Sport.