This is critical. It won't just happen, you need to make it happen and to do this you need a plan. Planning your activity and meals in advance means it is much more unlikely that your good intentions will come unstuck. Each Monday I schedule my walks (or other activities) in my diary just like any other appointment. When I was losing weight I walked every day (sometimes twice a day on weekends!) and managed it around my other commitments. Now that I am maintaining I do four to five sessions per week. If I have something on in the evening I schedule a lunchtime walk. If I am away in Melbourne for work (happens a lot!) with back to back meetings and an evening flight home, I schedule my walk for the morning and get up an hour earlier. Once you have written them down stick to them like glue. No excuses.
Use the same approach to meals if you a likely to succumb to a fatty takeaway because there is nothing in the cupboard. Write up a meal plan for the week on Sunday and do the grocery shopping the same day so you have everything available for the week, Choose quick and easy meals that take no longer than 30 minutes to prepare on weekdays otherwise it will be tempting to get that takeaway, particularly if you've had a tiring day.