While it's reasonable to assume that air units are faster than ground units, a difference of nearly a whole order of magnitude is too extreme. In fact, we have evidence that ground units aren't that slow -- Webinar only took a couple of turns to find Jillian (at a location that had to be close enough to GK to be plausible for somebody who really had escaped and travelled on foot).
Why on Erf would he choose to put the dwagons where they could be attacked by air units and ground units if they were discovered? (Putting them far enough away to guarantee non-discovery was almost certainly not an option. They had to hit enough targets to take out Ansom's siege train in two turns, which limits the amount of move they could reserve for their final retreat to their hiding place. Note that Parson's original plan repeatedly references the issue of the dwagons being able to reach all their targets; the revised version of the plan did not differ in that regard.)Assuming the Woodsy Elves to be fast units and Gumps to be slow, Parson could have hidden the A-dwagons in the woods near the lake. Not over the lake, and slightly further "away" from the Donut of Doom. Ansom used the bats (as Parson predicted) for the search, so as not to waste the other unit's move points on a search effort.
Huh? He did send Jillian on the hunt, which wasn't exactly a safe undertaking (and, in fact, she did nearly get croaked).