I have reduced around 8 lbs in 3 weeks from walking alone combined with healthy and measured eating.
What ever time you choose for walking in whole day, but always try remembering that walking should be at a pace which you feel comfortable with and without straining yourself . Gradually after a few days your speed for walking will increase and you will feel and see the difference in your energy level and reduction in weight.
Too fast and excess walking in your initial days can put you totally off track as you will feel disheartened and hurt your feet . exp your shin. Walking has to be at least 25-30 mts at one go to gain any benefit as initial 15 minutes only glucose burns in body only after that fat starts to burn.