Originally Posted by
fumje
Not that it's a surprise

, but you've hit it pretty spot on for the break-even point of walking distance. Looks like it's about C25/C28 — see below.
However (overdoing the calculations here, now), in terms of average time break-even, I believe the point would be closer to the actual moon buggy gate (about D9), given the train departs about five times as frequently as the buggy departs. But from wherever you start, if you choose wrong, on average it's about five minutes difference (assuming a conservative walking speed of about 2 miles per hour / 175 feet per minute).
C25/28 to C9 + C9 to train + train to B45: 955ft + 1110ft + 776ft = 2881ft
C25/28 to buggy + buggy to B45: 950ft + 1872ft = 2822ft
(google maps)
Should have enough time to pop into LH or TK for a quick bite / drink, assuming you're fine boarding after first called. Might get some pushback if you try to take a beer to the gate though, if that's what you're thinking.
no push back at all, if you pour it into a coffee cup to go with a lid! A hack everyone should know! However I never looked for one at IAD TK, AF or LH I know UA has them.