Notes

Specificity refers to an exercise, activity, or workout that targets your specific training needs. Specificity can be applied to parts of your body (like arm or leg strength) and areas of fitness (like cardio endurance or flexibility).
Progression includes increasing both the duration and intensity of a physical activity gradually over time. Attempting too much activity before you're physically ready can lead to injury. Nobody can go from couch potato to marathoner overnight. That's why it's important to plan short-term goals, build on your accomplishments, and make slow and steady progress each week.
Overload means working your body harder than usual in order to improve how it functions. Exercising at a higher level forces the muscles and systems in your body to eventually adapt. When you overload your muscles, they contract more, which increases their size. Your muscles’ cells will also develop additional mitochondria, helping them provide more power in the future.
Reversibility means that when you take a break from your exercise routine, your fitness and skill level will actually decline or reverse. Have you ever taken a break from your exercise routine, only to return and find you had difficulty reaching the same performance level? That is the principle of reversibility at work. When muscles go unused for an extended period of time they shrink and weaken, a process called atrophy. To sum it up, "use it or lose it."
Tedium suggests including a variety of activities in your workouts to avoid boredom and overuse injuries. If all you ever do is jog, you should add some other activities to your routine. This will provide a change of scenery and variety in your workout and help keep you from losing interest, not to mention saving wear and tear on your knees. Recurring, repetitive motions can sometimes lead to overuse injuries.