Picture the life of a male lion. Most of the day, they lie around doing nothing, letting the females make the kills. They watch the young or patrol the territory. They may spend as many as 22 hours a day inactive.
When called to action though, perhaps to fend off a rival, defend against hyenas, or help with a big prey, they act ferociously, if even just briefly.
They have incredible power, able to jump 10 feet off the ground while holding a 400 pound animal in their mouths. Should they do too much exercise though, they grow easily exhausted. Their girth makes them ill-suited for continuous activity. They also must conserve their energy for any possible famine.
Many trainees do too much medium intensity cardio. They avoid either working brutally hard or taking it easy. As such, they never really put in the effort required to bring about exceptional results.
Progression requires more work in the same or less time first and foremost. Trainees often lift weights 3-4 times a week and do their long cardio on the off days. This leaves them in a state of never really feeling rested. Their minds and bodies have a perpetual ache. Never getting a break limits their effort and motivation.
Act more like a lion. Make your tough, brief exercise even tougher and briefer. Make your long and easy exercise even longer and easier or just rest. Turn off days into true rest days. If you want to be strong and muscular, go this direction.
On the off days, going for a long jog will only slow your recovery. Many joggers have a tightness in their legs that lingers during the week. This will also beat up your joints with the constant pounding of your feet against the ground.
All exercise forces some system to recover. The central nervous system, which acts as the command center that directs muscle throughout your body, can get exhausted from too much exercise. So even if your muscles seem ready to go, other systems may break down if you train too often. Those whom get too little sleep often fail to improve, as their central nervous system operates poorly even if their muscles feel fine. Too much exercise affects you similarly.
You can still perform cardio but need to get away from the traditional route. Factor it in without taking away from lifting. Consider going for a brief walk on your off days to fulfill your need for daily activity. If you truly need to jog, keep it easy and for no longer than 20 minutes or treat it like a tough workout and plan accordingly. Pay attention to your body and avoid overdoing it.
Intervals may work instead. Although intervals done the right way hit your fast-twitch muscle fibers, the limiting factors seem separate from weightlifting. This may interfere less with your recovery from weightlifting since the duration is so brief and it affects your systems differently. Be careful though and pay attention to your body. If you need complete rest, take it.
You could devote one or two days a week to full-body lifting, then train intervals or walk on the other days. You will find you can put your all into every workout. You make progress instead of just going through the motions for the sake of staying busy.
You should feel fresh whenever called upon to work hard. A state of weakness at all times will stop you from putting forth that extra ounce of effort that can make the difference between steady progress or doing the same for years upon years.
If you continue working at only a medium intensity, you probably should work out as often as possible. You will gain moderate health benefits and some fitness. Cutting down on off day cardio only to continue dogging it would just reduce your already meager results even further.
Exercise is catabolic. It wastes away tissue when pushed too far.
If you truly embrace the path of hard work then you have to really kick ass. Train heavy with basic exercises and seek to add weight to your lifts. Push your intervals until you gasp for breath and feel the burn that seems to dissolve your tissue. Earn rest days and reward yourself with a period to rejuvenate.
Avoid long, medium cardio on your rest days unless you need it for sport. You will get more out of your lifting and intervals. Have complete rest days when needed. You will come back better, and also have time to enjoy and succeed in your life away from the gym.