Avoiding overtraining is one reason to train with good heart rate monitor. In some days you must run much slower than other days because of stress, lack of sleep and rest. When you hit your upper limits, it's a sing to slow down.

Suddenly reduced HRV is another sign of overtrtraining. Good heart rate monitors (some Polar models with a strap) can do Orthostatic Test and give even more accurate information than heart rate.

https://www.polar.com/blog/heart-rate-variability-and-orthos...

Mountains, perhaps thanks to summit fever, really seduces people into overreaching, and cover up the clues at the same time. "I got to the summit, so it's all good!" you think.

I really can't recommend "Training For the New Alpinism" enough. It really disabused me of that notion, and after a year or two of conscientious work I can hike the same trails with a smile & easy breathing, faster than I once could dripping sweat & suffering.

A sport heart rate monitor is also extremely valuable.

Heh, I've actually been doing TFNA for about two months now. Just finished the "transition period."

If you're having these days where your heart rate is elevated for weeks after, during your transition period, you either started off with way too much work or you're not tracking your work honestly. In just a month or two during the base period you will wind up doing more than double what you're doing right now, so it's very important that what you're doing right now is really quite easy to sustain. Also, heart rate zones! Make sure you are following them!

About once a month I take a weekend to go to my absolute physical limits, and then take a half week or so of rest. It's probably not healthy, and it's certainly not good training, but it sure is fun. Not many people get to say they've seen the enchantments or deep into the goat rocks wilderness in the dead of winter.