I saw Jimi Hendrix on a video once jamming for lord knows how long, at least 10 minutes, at least. I said to my mate who put it on "that was a mint track". To which he replied with a blank look on his face "what track" "eh the long one just now" "no mate that was an interlude, he was just playing as it came to him then, that was not rehearsed or written on paper as a track".
I admit i thought that was a desperately cool skill. It really sounded like a mint track someone spent ages writing the music for.