Aww hun your doing a big walk every day, thats awesome, i don't know how you find the time to do that!
Is there any way you can maybe cut the length of the walk in half on say 2 days a week and come home and do something 'aerobic', i have always been told walking is the best form of exercise so i think your doing a wonderful job!

I am turning my house work into exercise or 'play time' with the girls, we put music on DD2 in the jolly jumper and i jump along with her or dance with DD1 in the lounge to the wiggles, jumping up and down gets the blood pumping. I don't know how the wiggles do it...

Keep up the great work hun.