I started my doctorate before I retired, though I had cut back on my responsibilities at work and in the home. At 56 I was going great guns until I hit 2 problems, first on the course then at work, so I put off completion until I retired. It never happened, and now at 65 I know that even had I started the data collection for my research as soon I retired, I don't think I'd have completed. I threw the towel in and last autumn picked up another Master's for what I had done, and I started binning my many, many copies of research papers last week.
Strangely on Tuesday I had a conversation with a friend who is in a very physical job and cutting back on hours substantially. We agreed that once you stop working full-time, your energy levels drop markedly, and so goes motivation.
To complete 6 yrs part-time you need lots of interest from family/friends, and lots of practical and emotional support, not just from home, but from uni as well. Very good luck x