^ Try working some road work into it, I'd bulk up the bag work a bit aswell. Punch out drills etc, don't feel like you have to stick to 3 minute rounds with a long brake or what ever... in our gym we normally do 1-1:30-2 minute rounds on the bags with a 10 second brake for 20 rounds all out punching with a set combination or technique for each bag.

When you say stamina, do you meen how long you can perform or how hard you can perform? I swear by sprints, do a distance you feel comfortable with (I run for about 5-6 miles at a decent pace before I tire, so 4 miles is comfortable for me) and look for areas that you can sprint on your root. There are loads of stairs, hills and long streets etc that I sprint up and down at different speeds and amounts depending on their length and steepness. Experiment and see what works for you.