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We usually send Wally on such errands

And HE said that Evil Black rifle was just for Prairie Dogs ;D

If you read the links I posted about nose breathing during strenuous exercise and running, they explain why a mouth breather will feel a decline in performance at first when learning to breath again from the nose predominately.
Under heavy enough exertion or when your at or past your aerobic thresh-hold, you'll start mouth breathing whether you want to or not.
We start out as babies breathing thru our noses, somewhere along the way, most lose some or all of the ability. Re-learning something that comes so natural as a child after years of doing it differently is no easy task.
Your normal pace as a mouth breather may drop off, but you should see a increase in stamina/distance albeit at a lower speed. Distance and effort builds stamina. The longer you work at a slower sustained work load the harder you can go in short bursts and the faster you recover afterward.
Reason why sprinters/bikers do interval training. All out, recover at a moderate pace, then go again. You keep bumping up/shortening the recovery phase, the longer you can go all out.