I personally don't think you need to obsess over data (even though I'd admit I enjoy gathering and analyzing it). I have used a lactate meter, heart rate monitor, and plain old sensory data (i.e. effort relative to the distance being raced or workout being run).
I don't know if it was sirpoc or another poster that mentioned once you get a familiarity with the effort you are putting out in a workout and how that correlates to your blood lactate readings, you can use your sensory data to execute the workouts to a pretty optimal level.
For example, I live in Florida and know how my expectations and training during June through September will need to be modified. I do less volume in the workouts and not as long of a long run, yet inevitably feel more of an internal training load stress because of the heat and humidity.
I have seen rTSS discussed and understand it to be a metric used to quantify training stress/stimulus, which I think is great. Daniels, in his second addition (and possibly updated ones) included a table which assigned a point value based on duration of the run and intensity. I also think just learned experience and an objective assessment of your training and performance is equally viable. You know when you're pushing beyond what is sustainable and repeatable, it's just a matter of discipling yourself and having confidence in the approach to see it successfully through!
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