It's really interesting that you want to go back, educate yourself, and further your career, but at 56 I imagine there are other things that could be more fulfilling than 4 rough years of studying + residency. Children, grandchildren, wife/husband, or if divorced trying to find a new partner and enjoying retirement, vacations, being a parent/grandparent, doing hobbies since you have more free-time - painting, fitness, meditation, gardening, hiking, etc...
I feel like you're getting to the age where in general most people try to reduce their hours, so they can start to relax. I'm not sure it is worth years of lost free-time or the years of debt.