Cardio can crush a gazillion calories in a single session, but building muscle through resistance training can be a much more enjoyable route. Muscle is metabolically active. It requires energy at all times. What does that mean in practice?
More lean muscle on your frame equates to a greater proportion of consumed calories being directed towards just maintaining that muscle, versus being stored as fat or burned for energy.
You should prioritize building lean muscle. Resistance training promotes this signal in your body. Running, unfortunately, does not.
Not only that, but your body tends to adapt quickly to excessive cardio. It becomes increasingly efficient at covering the same distance with less energy (fewer calories). When food was scarce, and you had to travel long distances to find calories, this was a great feature. You could go further with less energy. Today, we simply don’t face this problem. We face the problem of having too many calories.
All else equal, a 5 km run today will burn more calories than the next 5 km run. You burn increasingly fewer calories the more proficient you become at running, and the more your body adapts to the activity.
This is important: your body will not prioritize building muscle if you are constantly running. Why would it? Muscle is heavy, and you don’t need a lot of it to run long distances.
Take a close look at a long distance runner vs a 100 m sprinter. The marathoner’s physique is extremely slim and light, while the sprinter is packed with lean muscle. Both groups have low body fat percentages, but sprinters have far more muscle mass.
I’m not trying to convince you not to run. If you love it, and it works for you, fantastic. However, for many people, resistance training may be more conducive to their goals. If you’re like me and you enjoy eating, but hate long, steady-state cardio, picking up the barbell and building some muscle is the obvious choice.