I would be shocked if the answer were not Y = 1.5X + 2. The y-intercept has to be 2 if you want to model the situation as a linear relationship. Because of the wording in the first sentence of the problem, I just assumed that the increase is 1.5 miles each week even though he jogs every day.

I think that it's great that your DD is giving it so much thought and considering all the possibilities - it's the hallmark of a strong mathematical mind.

I also think that once your DD receive more exposure to how many problems in science/social studies are modeled with the x-axis representing time but X=0 not the beginnng of time, it will make more sense. I guess I have always found it helpful to think of X=0 as the point before the change since the slope is meant to model the rate of change. For example, X=0 can be the year 1945 if you are plotting economic data regarding the increase in housing construction post WWII.