Ask a buddy to do it with you! You get that feeling that you don't want to let them down which encourages you to stick to it, and knowing someone's there with you can kill off any anxiety you may or may not have building up that's causing you to just say nah.
And don't think you have to pick something "serious" (major airquotes there because most workouts can be done casually); Even just going for a walk to a park is good for you. You don't have to "hit the gym" or pick a "workout routine". Any step towards better health is a good one. You can consider the other stuff once you've taken whatever steps you're comfortable with right now.
Don't let yourself get to out of shape. It gets mega harder to get back in shape the older you get. Trust me on that, I know the hard way. Araragi Kun has the right idea, just taking the dog for a long walk a couple of times a day is good for you. Plus the pooch seems to like it to