Went an hour to see what I'd get at the arena lap. It's a little over a mile per lap, 1.06 or something. There's a sharp corner at the bottom of the hill each lap that ruins the fun a bit, but it is secluded. I did 24mph for 10mi of it and 1:03:56 for a 40k overall. It's basically a false flat down to the corner, then a steeper false flat up the other side. All the "hour" style KOMs faster than me were done by training group rides of some pretty fast folks. I managed 7th I think.
Not terrible. Pacing is tough because for the corner you get at least 10 seconds of braking/coasting each lap and going up the hill each lap you go "over" your target. Given that, you wind up with about 6min of "zero power output" time for an hour. 15 seconds done 25x.
I would assume on a flatter route without the corner every lap, or the hill, I would be faster. You can't say that you'd gain the distance you would cover in that 6min, because you'd only go marginally faster using that power at that speed......but it's something. I'd say you'd probably go 6mph or so faster not braking for the corner. So, 6min at 6mph........0.6 mile extra. Perhaps.