The walk home includes a tiredness that the outward journey could not estimate perfectly.

Setting out, you plan with the energy you have, not the energy you will have. The outward leg feels like a fair sample of the whole trip, so the distance back gets costed at the same rate, and it never is. This is a general shape, not a walking one: the second half of any effort is done by a more tired person than the one who agreed to it.

When you are deciding how far to go, budget for the return separately. Leave a little in reserve on the way out, and treat any plan that only works if you stay fresh as a plan that needs shortening.