How are you going to learn to box if when you get put in the ring to spar your so tired after 15 seconds that you can't think straight?

You need a certian level of fitness just to learn the technique. Forget standing still and practicing single shots and individual stances and defences, yes you need all that but unless you can put it all together whilst under attack its all worthless. only way to do that is to spar... only way to learn from sparring is to be fit and agile.