I find that I can walk up to most restaurants in the resorts, and many in Epcot without an ADR and get seated within 15-20 minutes. Some of the more popular places like Le Cellier, Teppanyaki, or Ohana pretty much require an ADR.
I've never gone to Whispering Canyon at lunch time, but I would guess you could probably get a table pretty quickly. You could also try stopping at Guest Services at one of the parks to see if you can make an ADR before heading over there--they may be able to give you an indication of how busy it is.