有一天: past as well as future?
Yes, `one day' can certainly be used to refer to events in the past as well. 

A better translation of "One day, the boy went for a walk" could be "有一天, 那个男孩外出散步", or less literary, "那个男孩出门走路". "去了走路" is not grammatical, "去走路了" and "走路去了" are grammatical but do not fit in here.