Go longer, not harder.
I knew a guy once who could muster incredible strength in his arms and legs. He could lift stuff, push stuff, and outrun anyone else. The thing is, though, while he was physically strong, his endurance was terrible. He could only keep his strength up for about five minutes before getting tired and having to rest for twenty. If you’re regularly on your feet and moving, work in a physical profession, or are yourself an athlete, you should know how important it is to not only be able to exert strength, but keep that strength pumping for long stretches. Runners, in particular, have need of this skill, which is why runners make use of tempo runs to build endurance.
As opposed to a high-intensity sprint, the goal of a tempo run is to maintain medium-to-high levels of exertion for a long period without tiring yourself out. Experienced tempo runners have a good feel for their bodies, and are able to endure their hearts pumping at high speeds. Some tempo runners can withstand 80-90% heart rate for up to an hour. Even if you’re not a runner, learning to tempo run is a great way to get comfortable with extended exertion.
There’s no such thing as a one-size-fits-all tempo. You’ll need to experiment a bit in order to determine where your middle ground lies. Experts suggest running at a pace that’s much faster than you would normally run, but still a good few miles per hour slower than a full-on marathon sprint. A good means of finding your groove is the ladder method; warm up with a leisurely run, then run for ten minutes at where you think your 50% capacity is. Take a break for a minute, then go for eight minutes faster than you were before, then six minutes, then four minutes, then two, each progressively faster. You’ll learn a lot about your body’s limits, as well as begin building endurance.
Remember, big, strong muscles can’t do anything if you don’t have the endurance to lift them.