Do it now, put your shoes on, open the door, walk for five minutes one way then turn and come back. Walk at the speed you’d go if you were late for an appointment somewhere that was a ten min walk away, you need to get there in five! You should get a little out of breath and slightly hot.
If you can walk somewhere, and you have time, then do it. Leave the car where it is, forget buses and trains, forget bikes even, get out there on your feet. You’ll feel so good, there’s nothing like moving using your own power You’ll have a bit of time to think, breathe fresh air, see people and say hello; it’s fantastic.
If you have a bit longer each day go for a longer walk but intersperse it with a few of those faster ‘late for appointment’ bits, or stop somewhere and do a few star jumps, or get a skipping rope and skip for 2-3 mins when you get back, or do some sit ups or press ups, squats or lunges.
15-20 mins walking a day is a whole lot better then zero mins, and if you do that every day you’ll end up doing over two hours in the whole week.
If you actually would like to start running after a few weeks, if you get the outdoor fitness bug, then start slowly, add a few short jogs into your walks then gradually lengthen the jogs til you can run for 20 mins, build from there.