We often go without pre-departure ADRs and have pretty good luck, but there are only two of us, and we are flexible on where and when.
Not walk-up, but less planned: Front desk/reservation concierge can help, even same day, for finding open ADRs.
Walk-up: For the very popular restaurants (e.g., Le Cellier, Cali Grill), walk up may be impossible, even for off hours. For other places, if there are only two of you, you can often be seated in a reasonable amount of time (15-20 minutes). It helps if you are willing to sit at a bar (if there is one, including at Cali Grill), and not going at the dinner rush makes it easier. If there are more than two of you, it gets more difficult.