The last year of college when I was thinking about what to do in the future, it occurred to me that being an unreliably happy low energy person, I wasn't going to ever really enjoy life that much until I figured that out, solved that problem. Eventually, I found running every day made me reliably happy and productive and I had a good ten years until that stopped working. I couldn't run as far, started having chronic injuries, started becoming less happy, less productive. Was I exercising too much, too little, I wasn't sure. I became interested in figuring out reliable indicators of undertraining vs. overtraining (the scientific research on which is still very unclear). I've also always enjoyed drinking, smoking, and occasional recreational drug use. I always found it interesting that some very healthy people seemed to engage in those activities to excess for many years without health repercussions. Sigmund Freud was an unproductive depressive until discovering cocaine and relied on it for at least twelve years (source). Certainly, some people have serious consequences from chemical abuse/addiction. But the reality is many people utilize drugs of many different kinds (antidepressants for the depressed, amphetamines for the ADD/ADHD, etc.) to enable them to live functional lives or just to better enjoy/experience life.
This got me on to the idea of the utility of physical and chemical exercise. Drugs act on the body, stimulating, causing changes in functioning, not entirely unlike physical exercise acts on, stimulates, causes changes in the functioning of the body. Certain types of exercise, in certain doses can cause alternately better health, injury, or even death in the moment or over the long term just as certain types of chemicals in certain doses can cause better health, injury, or even death in the moment or over the long term. The art of physical health is the selection of the right type of stimulant and the right dose, consistently, avoiding under stimulation or over stimulation, undertraining or overtraining. While I think physical exercise is the healthiest and safest type of stimulation, and should be the primary means of maintaining health, I think chemical exercise can be used as well supplementally, to maintain health, to enjoy life.
The human body has two energy systems, aerobic (endurance; low intensity) and anaerobic (strength/speed; high intensity). Activities like walking, hiking, light jogging, housework fall more under aerobic activity, whereas activities like short distance sprinting (100m, 200m) and heavy lifting far more under anaerobic activity. Somewhere between 400m and 800m sprint distances (probably about 525m) appears to be the sweet spot of aerobic and anaerobic system balance (source). If you look up youtube videos of athletes that run below or above that you will see the different body types that a more anaerobic or aerobic fitness bias will yield. (In fact, untrained athletes have more anaerobic power than long distance runners). Of course the physiological response of 525m in a trained athlete is likely different from that of an untrained one. For example, the anaerobic response in untrained athletes to a 1500m run is about ten percent greater. Another research study concluded 75 seconds of all out sprinting is the best balance of aerobic and anaerobic energy systems (source). Coincidentally, an elite athlete's 525m sprint time will be around 75 seconds. However, the vast majority of the population is much slower. This prompts the question if you are looking to design an optimally balanced aerobic/anaerobic workout and are not an elite athlete, do you do 525m sprint repeats, 75 second sprint repeats, or something in between? Since untrained runners use more anaerobic energy travelling the same distance as trained runners, I'm inclined to select 500m-ish interval lengths as 75 second duration intervals will be too short (distance-wise) and thus too anaerobic heavy.
In the absence of more definitive research/answers, selecting the right exercise amount, frequency, and type (aerobic or anaerobic) gets back to the art of physical health question. And like all art, that means you are left to experiment for answers. Beyond picking the right training distance/duration, you have to determine the right training frequency, rest time. The most objective measure of training effectiveness is speed and volume. If your exercise volume and/or speed don't increase, or if they decrease, then it's more likely you are overtraining. (Personally, I've found sometimes my volume ebbs and flows, so I would not necessarily associate occasional lowered volume alone to overtraining.)
Beyond that, I've also experimented with various physical markers (like the feeling of soreness in specific parts of the body, however minor) and have gone through many different markers in an attempt to find additional reliable undertraining/overtraining metrics. My current metric is footpad soreness in both feet. For training, I'm doing one session of 500m sprint repeats, walking in between sprints, every 72 hours (although depending on your recovery/fitness capacity you might work better with a shorter or longer rest period; I was originally doing every 48 hours but kept getting sick, and my exercise speed/distance was not increasing consistently). As soon as I can't run a sprint at my current training pace (I use a GPS watch to monitor my pace) or I reach my soreness metric, the sprint repeat part of the exercise session is over and then I just jog or walk until I reach my soreness metric. If I can complete 10 sprints at my current pace in a session, I lower my pace the next session. (If you are new to running, be conservative with your sprint pace, better to start slower than too intensely and get injured or sick. You also want to pick a pace you can complete 10 [or close to] 500m sprints at.)
My current regime...
Physical exercise (sprint session every 72 hours and light jogging, hiking, or walking otherwise) whenever I don't footpad soreness (in both feet). Avoid chemical exercise if I have footpad soreness (I generally avoid drinking more than once a week).
Footpad soreness = rest/cease exercise
Hunger + footpad soreness = eat
Otherwise, be/stay active (with one sprint session every 72 hours)
As to why strength training exercise is not enough, HRV (heart rate variability), which is metric that is predictive of mortality, improves more with cardiovascular training (source). Strength (anaerobic) training still has value as it improves aerobic capacity via better efficiency (source). However, I think that an HIIT approach to training (alternating between high effort and low effort) may effectively develop both anaerobic/strength and aerobic/endurance capacities in one exercise (source). To point out the obvious, the less in shape you are the less you can do high intensity exercise and the more you should rely on a higher percentage of low intensity exercise. However, it is essential that you both engage in hard and easy exercise. The former conditions your anaerobic/strength system more, the latter conditions your aerobic/endurance system. Following only a high, medium, or low intensity exercise approach leaves you less or undeveloped across one of your systems, and also more likely to overtrain in the one system you favor (often to that system's detriment). If you are were only going to do one type of exercise, then aerobic exercise is best, but people who do aerobic and anaerobic exercise are the healthiest / have the lowest rate of death (source).
Heart Rate Variability
The amount of time between each heart beat varies from one beat to the next. The healthier you are the more it varies. Sickness and aging cause a lowering of heart rate variability, i.e. a more regular, clock like heart rate. Exercise improves heart rate variability (as long as you don't overtrain). The Palo Alto longevity project which has a million dollar award for scientists that can reverse aging in an animal uses heart rate variability as it's main criteria for determining a winner.
With a $50 bluetooth strap such as the Polar H7 and a cellphone app like Elite Hrv or Hrv Expert you can easily track your heart rate variability and determine what works and doesn't for improving it. Taking a reading every morning of at least two minutes at rest is all you need to do to effectively track your HRV. The two numbers you want to track are RMSSD and SDNN. As important as increasing RMSSD and SDNN is the SDNN/RMSSD ratio, you want that particular number lower. Lower RMSSD, SDNN, or a higher SDNN/RMSSD ratio are signs of too much stress (not enough recovery/repair).
Recently scientists discovered that by connecting the circulatory system of a young mouse with an older mouse, signs of aging were reversed (new brain cells grew, the muscular skeleton system grew stronger, thickening of the heart walls was reversed, exercise endurance was increased) (source). The researchers discovered a protein called GDF11 was responsible. They have not done any human research on this yet but I am eager to see whether there is a link between GDF11 levels and cardiovascular stimulation (either from exercise or drugs/chemicals as this would explain why exercise and anti-depressants have been shown to be effective at treating depression, promoting neurogenesis).