This is tough to answer but I am a firm believer that just like a runner trains to run a half-marathon, a walker should do the same. They should commit to the same training time-line as a walker as they would as a runner. Could someone walk a half-marathon with little or no training..absolutely, however, you must look at the overall picture.
While cross training is a great thing, remember the Principle of Specificity states that in order to run, one must run. In order to walk at a race pace, one must train at that race pace.
Look at Usain Bolt...he runs every day in his training--probably covering 5000 meters in the course of a training day, but that does not mean he could race 5000 meters. The reason, we train specifically for the event we want to participate in.
And because the time frame to adapt to this type of training is unique to the individual it's difficult to say.
The average walking pace for most people is approximately 3 mph (20 min/mile), however, there are some walkers who can walk closer to a 5 mph pace (roughly a 12 min per mile pace) but they are few and far between. The reason, walking at this pace actually can be more difficult than doing a slow run...this is what the experts call "the break point" when a slow running pace is easier to maintain than a fast walking pace.