It's a bit of a tricky question because I can follow Dave's logic and I can play devil's advocate.
Given there are plenty of pork pie munching, lager swilling yobos who fight every weekend around the world the obvious answer would be you don't have to be fit at all.
However, if you want to fight well or be better at it then it behooves you to be in shape.
To begin with it was always harder to fight and knock out guys who were in phenomenal shape as opposed to slobs when I worked the doors. The fit guys seemed to be capable of taking more punishment and had faster responses etc.
Next we have to factor in adrenalin and stress which can absolutely tax someone without their being aware of it. Fit people again handle stress better than their unfit counterparts.
As for fights going ten seconds...yep, against one person maybe. The big one outside Camden Tube went way longer than that and there were plenty of times in Oz when I had running battles at clubs (for example chasing someone who'd grabbed money out of the till or threw a bottle through the club window)
So I don't know...I'd say very important if you want to make a living out of it or expect to do it more than most.
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