Christian:
This is a great and very deep question. You always ask challenging ones. I’m sorry I didn’t see this earlier but……
This has been the age old problem for coaches and athletes. How to quantify the training load and the fatigue that coes with it. Training load is a term that, while vague and defined in several different ways, is a concept that captures all the things you speak about: duration of individual workouts, intensity, frequency, and total volume. The best method for capturing training load that I am familiar with are incorporated in the metrics that Training Peaks uses. These are based on TSS (training stress score). The TSS is calculated using an algorithm which involves power output (running or cycling), pace (running or simming) or heart rate (as a last resort when these others are not available. The TSS is a function of duration and intensity and the time spent above and below what we call AnT (TP calls this Threshold or FPT-functional threshold pace or power). So now you have this single number, TSS that is a proxy for training load. The power and pace baed TSS is much more accurate that heart rate based TSS.
From other algorithms TP comes up with CTL= chronic training load which is a proxy for fitness or work capacity. ATL= accute training load which is a proxy for fatigue and TSB=training stress balance which is an approximation of your form on any given day if you need to perform well.
These were all developed by Andrew Coggan PhD.
Like all algorithms, though; they need good data to give good results. That is why power and pace give more accurate TSS numbers than heart rate does.
I should do one of our book club sessions on TP metrics.
Watch for an a couple of upcoming podcasts on using power meters for running and hiking. I think you will enjoy them.
Scott