Good, thats when you do the sprints, when your arms feel rubbery.. ill do a 1 minute round a of power shots, than a 1 minute round of sprinting(heavybag), that should do wonders for you.
It takes many months or years, depending on your background, to achieve boxing shape. Keep track of your progress and try to improve a little bit every week. It will be easier if you put yourself on a schedule and force yourself to stick to it.