I saw John perform this past July at the Grey Fox Festival (formerly Winterhawk). He was battling cancer then, and had a band with him. He would play his fiddle and sing a bit, and then fade to the back of the stage where he would sit down for a few minutes to catch his breath, then back to the front again for another bit of song.
In a conversation during the weekend with Mary Tyler Doub (producer of Grey Fox), she told me that it was the performing that energized him and gave him strength to keep on fighting the disease. Her eyes filled with tears when she talked about "that dear, dear man."