Dr. Atkins himself said - 'Exercise is non-negotiable.' He meant it! It is important to keep the metabolism boosted as well as to keep fit in general and tone-up flabby, underused muscles.
If you have always been active and done a lot of walking you should continue. If however like many of us you had a sedentary, even couch-potato lifestyle then it would be wise to up your activity levels. You feel better and usually lose weight faster.
Funnily enough I am on an exercise blitz right now to try to break a stall. I exercised for a total of two hours yesterday in three 'bursts' of 45 minutes, 45 minutes and 30 minutes. I have arthritic knee joints so I have to be careful. I will concentrate on walking today and some gentle jogging on the spot, later - maybe add a bit of vigorous dancing to fave CD tracks!
I will soon slow down to one hour a day, every day. Gotta watch those knees! But even a 20-minute effort every single day soon begins to pay off.
Good luck! It's hard to get started but once you do, you will even start to enjoy it.