I've been there. It comes hard. Just schedule a 30+ minute walk - put it on the calendar, just like taking the dog for a walk or taking kids to school. Time it. Don't stop early. Rain or shine. Once you get that down, move to something bigger. Add minutes, add an activity. I tied my walking to my kegels. Do it, get dry (drier). Have an objective. The first few weeks it took me an hour with a cane to get to the mailbox (a quarter of a mile round trip). Tonight, it took less than 10 minutes, junkmail sorting included. No cane.
I was told that my lack of willingness to do anything was in itself a sort of depression. That got me motivated. I'm not interested in being depressed.
I was only 57 at the same point in my progression from DaVinci to adjuvant RT. How hard would this be if you were 70? I gave myself a good swift kick in the arse. I needed it, and living alone, it was my own problem to deal with.