Stationary Bike by Stephen King

A middle-aged, out-of-shape man buys a stationary exercise bike to try to reclaim his health and control after a personal low, but solitary workouts soon turn unsettling: unexplained readings, intrusive sensations, and a mounting compulsion to pedal draw him deeper into an escalating psychological and possibly supernatural torment, forcing him to confront how far he’ll go — and what he might lose — to keep riding.