Fighting is a marathon, not a sprint. A boxer must stay in shape because boxing is essentially a cross between running and dancing. The energy you expend makes endurance training mandatory. There was always the debate when I was fighting about long-distance running. Some fighters said running long-distance could reduce muscle mass and others said that was a bogus claim. If you want to get into shape in 2019, take up boxing. Below are the endurance exercises you need to perform before you step into the ring:

Jumping Rope and Jumping Jacks:

Jumping rope and jumping jacks increase your heart rate and engage your arms. When you step into the ring, you will be flailing your arms, blocking and weaving for 10-rounds. Any exercise that works your arms and legs at the same time is massively beneficial.

Long-distance Running:

You need endurance to fight. This is the way to do so. You will also build your pain tolerance during your runs. Focus on 5-mile runs and ensure you have some 100-yard sprints in these runs. If you run on a treadmill, push the elevation to 6 degrees and stay above 7 miles per hour. Do not skimp on running for building your endurance.

Stairs:

In-line with jumping rope, running stairs will massively Jack your heart rate and engage your calves and thighs. Stairs can build your core muscles and also help you weave and dodge better. Running stadiums and buildings like Rocky did will really help your fighting game.

Getting into shape takes approximately 6-months to build up to. Remember not to overdo it, and take breaks when your muscles and tendons hurt. It is never worth it to push yourself too hard and lose time in your goal to head to the ring.