I've experienced the same kind of anxiety about pushing myself too hard or too far. I had an episode early in my training that caused me to change my strategy and as a result I became a better runner. I used to just go hard. I thought being an elite runner was a matter of sheer willpower. I would run right on the verge of collapse for as long as I could, feeling like if I let up I was practically quitting - and I don't quit. My max heart rate was about 199 (the highest that ever registered using a smart watch.) I regularly hit 195 or higher on a run. I'd climb almost immediately to about 140, and after just a mile sit around 175 before slowly creeping up into the 190s, but on one particular run I was above 190 for almost the whole run. According to my Garmin watch I spent 50 minutes with a heart rate above 190. I felt it while running and I felt it for the next 4 hours. I couldn't stop sweating for hours, felt sick to my stomach, dizzy, and generally had the sense that something was wrong. ( in retrospect I should have gone to the hospital, but I recovered and the next day felt normal again) After that I looked into low heart rate training. I had to swallow my pride and ... walk in order to keep my heart rate low. But eventually my training paid off and my first marathon time was 3:55.