Endurance brings us to higher life by elevating the heart rate. It takes us outside, in nature, where we come from and always return. Physiologically, glycogen stores which act as energy buffers get depleted. Our metabolism becomes capable of burning fat more efficiently, making us capable of using body fat as an energy source, while at the same time limiting weight gains due to depleted glycogen stores. But on its own, endurance is not enough. This is the case at any age: at our peak we need strong muscles. We need speed. The benefit of strength and speed go beyond the pleasure of being able to lift heavy weights, or feeling the thrill of an acceleration on the running track, or the rescuing power of quickly running from one airport terminal to another, or train station to another, to catch the plane or the train before they leave without us. As we age, we lose primarily muscle mass. One can define aging by counting the number of years since we were born. Alternatively, one can define age by physiological factors such as VO2Max, which declines as we advance into the decades. The higher we can keep VO2Max, the younger we remain physiologically. Another sign of aging comes from the loss of muscle mass. Likewise, to the extent you can retain and keep developing your muscle mass, work on your speed and reflexes over the years, your biological age will remain younger longer, allowing you to continue experiencing the best of life.
Several scientific studies show the benefits of strength and speed retention as we age. A team of researchers looked into the relationship between gait speed in older adults and mortality rates. They took 9 cohort studies between the years 1986 and 2000, across a total sample size of 34,485 adults aged 65 and older. Gait speed was associated with higher survival rates in all studies conducted. This makes intuitive sense: if an older adult can move at a fast enough pace for short time period without injuring him or herself, that person shows that most core functions are intact: muscle strength, coordination, sense of balance, general cognition and awareness of the surrounding environment.
Strength and speed also show benefits in preserving our DNA and the capabilities of our genes to express themselves. These types of exercise are sometimes referred to as high intensity interval training (HIIT). A study published in 2017 in Cell Metabolism highlights the effect of HIIT training not only in improving physiological functions, but also in preserving the integrity of DNA, specifically gene transcripts. The organic world, in this respect, is not fundamentally different from the digital, from computers. We are information. This information is stored in our DNA, in our mind and memory, and if this information ceases to exist or to be accessible and usable, we vanish. Having the ability to extend some level of protection to our DNA through natural physical activity seems to make the latter worth practicing. Once again, this is a lifestyle, combining the pleasure of engaging in sports with the avoidance of the displeasure of DNA losing its integrity earlier than nature intended.
Strength and speed training is therefore, unlike endurance, a series of highly intense efforts: pushups, planks, weight lifting, accelerations, speed drills, and so on. Doing this requires, like any other physical exercise, careful attention in preventing injury. Due to the extreme stress you are going to put your body through, it is crucial that you research how to warm up prior to going all out, and the proper technique for running at high speed, or lifting very heavy weights. Many have injured their backs by using an incorrect posture. When the injury damage is done, we can’t exercise for a while, at the very least. Carefulness is required and will pay off as much as the high intensity training itself.
There is, finally, a balance to consider. You might want to develop more speed than endurance, or more strength than a lightweight approach to sports. This comes down to personal preferences. One needs to take into account the performance goals and where our weaknesses are to attain a balanced and efficient body and mind. Too much bulk will make you heavy. Not enough strength training will leave you weak and unable to accomplish accelerations.
 Studenski, S., Perera, S., Patel, K., Rosano, C., Faulkner, K., Inzitari, M., … Guralnik, J. (2011). Gait Speed and Survival in Older Adults. JAMA : The Journal of the American Medical Association, 305(1), 50–58. http://doi.org/10.1001/jama.2010.1923
 Enhanced Protein Translation Underlies Improved Metabolic and Physical Adaptations to Different Exercise Training Modes in Young and Old Humans. Robinson, Matthew M. et al. Cell Metabolism , Volume 25 , Issue 3 , 581 – 592, https://www.cell.com/cell-metabolism/fulltext/S1550-4131(17)30099-2