The first 20 minutes are hard as hell. The second 20 minutes are hard as hell. The last bit (it better not be 20 minutes, or you're doing it wrong) is even harder.
I think I'd be very tentative with the pacing and aim for a solid negative split. With a 20k, you can almost go out to hard and screw it up and still hold on to a decent time. Not happenin' in a 40k, if you find yourself hurting after 10k and needing to back off, you're screwed
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"Your beauty is an aeroplane;
so high, my heart cannot bear the strain." -A.C. Jobim, Triste