Here is a study that gives some idea of the degree to which the majority of people who do not maintain a good level of fitness are harming themselves over the years:
The study of amateur older cyclists found that many had levels of physiological function that would place them at a much younger age compared to the general population. The study recruited 84 male and 41 female cycling enthusiasts aged 55 to 79 to explore how the ageing process affects the human body, and whether specific physiological markers can be used to determine your age. Men and women had to be able to cycle 100 km in under 6.5 hours and 60 km in 5.5 hours, respectively, to be included in the study. Smokers, heavy drinkers and those with high blood pressure or other health conditions were excluded from the study.
The results of the study showed that in these individuals, the effects of ageing were far from obvious. Indeed, people of different ages could have similar levels of function such as muscle strength, lung power and exercise capacity. The maximum rate of oxygen consumption showed the closest association with age, but even this marker could not identify with any degree of accuracy the age of any given individual, which would be the requirement for any useful biomarker of ageing.
"An essential part of our study was deciding which volunteers should be selected to explore the effects of ageing. The main problem facing health research is that in modern societies the majority of the population is inactive. A sedentary lifestyle causes physiological problems at any age. Hence the confusion as to how much the decline in bodily functions is due to the natural ageing process and how much is due to the combined effects of ageing and inactivity. In many models of ageing lifespan is the primary measure, but in human beings this is arguably less important than the consequences of deterioration in health."