I once asked a 65-year-old friend of mine when was the last time he had gone all out. He replied, “John it's been decades”.
Our bodies respond to the stresses placed on them by making a positive adaptation to handle that stress. Our skin becomes tan, our hands become calloused, our muscles become stronger, and our body increases its capacity to burn sugar longer.
If we do not place demands on our body our body downgrades its ability to handle demanding work. Muscle is metabolically expensive to maintain. If we do not need it we lose it, and our bodies become weaker. As a consequence we burn fewer calories, we lose flexibility, our cardiovascular system becomes compromised, we are more prone to injury, our immune system becomes weaker, and our bones decalcify.
How much demanding work is enough to cause a positive change? That depends on how far out of shape you are. From this article, Six seconds 'can transform health', comes this quote:
"A group of pensioners came into the lab twice a week for six weeks and went hell for leather on an exercise bike for six seconds.