Fitness and SD... interesting thread.
Just for context, I train Kenpo 5.0, which has us fighting at different ranges from contact to control manipulation. Kick-boxing-type tactics to bridge the gap, Kenpo sequences for in-fighting, various takedowns, and a large amount of groundwork out of BJJ, albeit with the mindset of getting back on our feet ASAP.
With that out of the way, imho, I think some measure of physical fitness is important. One may not be as conditioned as a professional cage fighter, or a 100-man-Kumite Kyokushin stylist, but given that combat is by nature, a relentless and brutal activity, it pays to be prepared for pretty extreme levels of exertion. Just my 2 cents. An opinion, nothing more.
True, the ideal scenario is to get it over and done with in 3-4 seconds, but if that doesn't happen, then what? Say, I've executed a Kenpo SD sequence,
and he's still standing, then what? Now he's seen what I can do, and things are going to get more difficult. Now, the fight's on, and to survive, I may have to last more than a minute, or maybe 2... or maybe more than that. Who knows?
Personally, I'd prefer not to take that chance. I want every edge I can get, and if it means being able to outlast the aggressor, so I can take him out of the game, and then have enough gas in the tank to practice my Nike-jitsu, then so be it.
Kind regards,
TCG