Overtraining, junk miles....other words that shouldn't need to be said.
You can run almost limit-lessly at near any age if you go slow enough and careful enough. Of course if you want to perform well within your ability and age, then you do want to run faster and or steadier.
To overtrain you would need to do a ridiculous amount of miles. Junk miles aren't junk miles if you are running within your ability to run, recover and run again, daily or more often than that.
You want to develop the ability to run 3 long runs per week - all quite strong, leaving yourself pleasantly fatigued at the end - not wasted.
Other runs can be short 'n easy for recovery. You should do alactic strides every week on a soccer field, under 10 seconds with full recovery.
You can do a fartlek run, go hard, but go by feel and don't dip into the anaerobic level much or at all.
Do 2 90-ish minute runs and 1, 2 to 3 hour run - make them steady. You can run tons of very easy extra mileage too - helps with recovery. Forget negative terms like junk mileage, pounding, over-training etc etc.
Run by time, not distance. Only read the watch and HRM when you are done. Love trail running and hills.
Keep it simple. If someone wants to tell you how long your foot should be planted each time or breath in a certain number of times per minute or have so many steps per minute, just ignore. Just run.
But do try to keep good form - hills and alactic strides will help with that.
Do 2 to 3 or 4 or 5 months of that, then the next step...