a couple of years ago i decided i would step in the ring and box a real USA boxing sanctioned boxing match.
i thought i was in decent shape up to that point. id been doing Crossfit since before it was the thing to do and you got the workouts online and did the WOD in your garage. i oly lifted along with my crossfit workouts. Before starting the boxing training, at 36 YO, i did a pretty easy mile with hills at 7:10. Not great, not bad, considering im 6-4, 230 at the time. Several times a year i would do the Murph workout with either a real plate carrier or a 25 pound workout weigh vest. it sucked, took me an hour, but i would do every rep.
enter the boxing gym and 3-4 times a week boxing training (45-55 minutes core excersises, bag work, foot work drills etc) . they said stop lifting weights, so i did. cut out carbs on the diet again, and went high protein and vegetables. In 4 months i lost 19 pounds. i felt terrific, except for that repeatedly getting punched in the face thing, but i got used to that. i had more lean muscle than ever, my cardio was incredible, and i was just all around built for speed and punching power; hands, feet, legs, everything.
Got in the ring in front of 1,000 people and won my fight by unanimous decision, with my opponent needing a standing 8 count once. the fight was exhausting, if i hadnt poured 4 month of training prior to it, id have fell down or passed out in the middle.
now to the running. during the training, i never ran at all. lots of jump rope though. 2 months after the fight, i did my yearly Tough Mudder, about the 8th one id done. 13 miles, in May, in Texas. i breezed through it without laboring, at all. 3 hours of running and obstacles, never started breathing hard. it was incredible, to say the least.
TL;DR: Give a good try to boxing training for an entire month, with actual proper diet, and see if it doesnt change your life. The cardio is where its at.