There are a variety of synonyms that can be used to describe "stays on the treadmill." One option is "perseveres," indicating that the individual is persistent and determined in their efforts to maintain their treadmill routine. Alternatively, "endures" may be a fitting synonym, as it suggests a willingness to withstand potentially challenging or uncomfortable aspects of treadmill use. "Continues" may also be a suitable choice, as it implies ongoing dedication and commitment to the task at hand. Ultimately, the choice of synonym will depend on the intended tone and context of the piece in which it is used.