I just checked to see what is available for dinner tonight for a party of 4. Except for BOG, Ohana, Sci-Fi, fireworks dessert parties, and some of the Disney Springs places that require reservations be made a day in advance, nearly everything else was open. And I did both a search for "dinner" times and "6:00pm" with equal success (except CRT, which does have an opening tonight...at 11:05pm).
Le Cellier, California Grill, 50's Prime Time, Hoop Dee Doo, Afternoon Tea at GF, Fantasmic dining packages at HBD and H&V, Club Villain....all had openings along with most of the other restaurants on property.
I don't know if I'd do walk-ups, but I think seeing what's available that morning or even just a few hours before you want to eat would be fine. The 180 days out thing is silly. I made ADRs 180 days out and then changed them on an almost daily basis for the next 6 months until I scrapped them all and went QS. Still had the BOG ADR to play around with, and a week out from our trip I was still moving that around pretty easily. Lunch. No breakfast. No, lunch again. Lunch and breakfast. No, just breakfast, but on this day instead. Maybe breakfast this day and lunch this day? It was pretty hilarious. And this was for Good Friday through the Wednesday after Easter, so a peak time.
Don't fret.