I like to take my time and enjoy the 5K and 10K: I'll run-walk between photo stops, but I make a LOT of photo stops - so I'm out there longer than on my training runs, but putting in less effort. I tackle the half and full about the same: extra walk breaks if I feel the need, push the pace in the second half if I feel up to it, make sure I enjoy the experience and don't regret not stopping for some pics.
Is this my cue? Ran a 20-minute PR in the WDW full, as part of Dopey, from a back corral. Didn't "stumble into it": I trained well and ran smart by sticking to the tangents as much as possible and not wasting energy dodging and weaving in congested areas. It very much IS possible!