What motivates Colin to practice walking every day?

Colin is motivted to walkin the garden because it once belonged to his mother.

"My mother?" said Colin, and he looked about him quietly. "This was her garden, wasn't it?"

"Aye, it was that!" and Ben Weatherstaff looked about him too. "She were main fond of it."

"It is my garden now, I am fond of it. I shall come here every day," announced Colin. "But it is to be a secret. My orders are that no one is to know that we come here. Dickon and my cousin have worked and made it come alive. I shall send for you sometimes to help—but you must come when no one can see you."