It is a matter of preference what time of day it is that you walk. If you are planning to walk outside, it'd probably be best during daylight. I walk on my treadmill at 8:30pm or a little later. I walk for 45 to 60min a day. Since June 1, 2012, I have lost 42lbs from exercising every day. I'm not on a diet plan and I eat most of the things I want without guilt.
It is important to get some cardio exercise for at least 30 min per day. You can walk 2 x 15 min walks during the day, 3 x 10 min walks a day as long as your exercising 30+ minutes a day, you will see difference in your level of fitness and quality of life.
If you are getting tired after dinner and falling asleep, you should consider eating dinner earlier and exercising after dinner. It will help burn off the calories. I was doing the same thing as you - eat dinner, watch tv and fall asleep. I gained 20lbs in 6 months by doing nothing. It is perpetual. The less I did, the more tired I was.
