'Addiction' to exercising is currently classed as a 'behavioural addiction' whereby a person becomes 'hooked' on the euphoria and pleasure that it provides (to some individuals).

It's known that heavy exercise can generate the release of hormones known as endorphins - which play a major role in the body's inhibitory response to pain, and which have effects that are in many ways analogous to opiate drugs.

Nevertheless :

As of 2016, the mechanisms involved in the development of an exercise addiction, associated with the transition from healthy committed exercise to compulsive exercise, are unknown.

Source : Wikipedia

Due to the lack of a precise definition, it's not possible to give a figure for its prevalence in society. Some studies have put the figure as high as 40% for regular gym members.

Further technical information : Int J Environ Res Public Health.8(10): 4069–4081.

Also see : Overtraining syndromeplugin-autotooltip__plain plugin-autotooltip_bigOvertraining syndrome

"When prolonged, excessive training stresses are applied concurrent with inadequate recovery, performance decrements and chronic maladaptations occur. Known as the overtraining syndrome (OTS), this complex condition afflicts a large percentage of athletes at least once during their careers.