Interval training, tabata circuits, plyometric exercises, sprint drills, bag drills, and pad drills.

Run a few miles at least twice a week.

Don't go crazy with lifting weights. Obviously it's good for getting stronger, but try to focus more on building power and explosive strength with stuff like medicine ball slams, tire flipping, kettle bell exercises, and high rep bodyweight exercises like pushups, pullups, dips, situps, rope climbing, etc.

Also important to spar if you can. It's really hard to replicate with excercises how tired you get from actually going 70-80% in the ring. Best thing you can do is get rounds in. It will really help your stamina.

If you can't spar, shadow box and do high-intensity bag work. Really try to wear yourself out. You can't just go through the motions. You really have to dig deep and push yourself to your limit when you train.