When I was in high school, a student band director had been working at my school for a few months. One day he was talking about individual practicing and how to become a better musician. He said that from his experience in becoming a better musician, when he was younger he started practicing only 20 minutes per day. Then he said as more time passed and he began building his skillset he added more time. 20 mins -> 40 mins -> 60 mins -> 120 mins -> and on and on. This was over months and years.

Another experience I had in a music program was right before I started college. I was attending a national honor band and one of the directors talked about practicing music while in college. He stated that you should practice 20 mins then study for another class or topic then practice 20 minutes again, rinse and repeat.

I suppose, it seems that if you want to build yourself up or shake off the spider webs or basically start from scratch, you should start slowly to (re)build your muscles. Would it be wise to attempt to run a marathon without having methodically trained your mind and body for the needed endurance in the face of that difficult challenge?

You don't have to start with 20 minutes. Maybe 5 or 10 or 15. Interval training is the way to do it, IMO.