you have the nerve wrote:
Boxers are way, way, way fitter than runners. Boxing is very much a cardio sport, and competitive boxers have insane cardio.
I'm not even a good boxer (15 amateur fights) or runner but I've run / won road races with 16:30 5ks while training. I've seen a guy in my area run 9:40s for 2 miles while training for the golden gloves.
A state or regional class amateur 3-round fighter typically runs 3-5 miles a day on top of another 2 hours of training including 15 - 20 minutes of jump rope. A good pro fighter is doing more like 5 - 8 miles several times a week.
Boxing rings are absorbent and absolutely drain your legs. Moving around the ring for 3 minutes feels like running 20 second sprints with 20 second jogs in the sand. I've seen excellent runners gas out from one or two minutes of sparring. Sparring might be 4-5 rounds once or twice a week (more leading up to a longer pro fight). In addition to this, most shadowboxing is done in the ring.
The pacing of fights / sparring is incredibly taxing. You explode, rest, explode, rest, explode, rest, ... plus your dealing with unwanted physical pain in addition to the self-induced cardio-type pain. Running feels easy afterwards.
I'd imagine most competitive 152 lbers and below -- 170 max before weight cuts -- can run sub 20. Those who do a little speed work would have relatively little difficulty breaking 18 minutes. I think a sub-15 5k is special.