Timing of speed work?

So, after much rejoicing following IMLT (mainly beer and sleep), it’s time to get things going again. One thing I always wrestle with is transitioning from early season sessions where I’m getting my legs and lungs under me again, and harder, more intense efforts. Historically I am always guilty of loading on too much heavy intensity sessions too early in the season and then managing those strains and stresses the rest of the season. So THIS TIME it will be different.

For those of you self-coached out there, how do you know when it’s time to start introducing harders sessions into your workouts - especially on the run. Other than “later” in the process. Are there stress tests or other metrics you use with confidence to know your system can bear the stress of higher intensity workouts?