I wouldn't. Gyms are the classic example of a purchase in which a long commitment is unwise, for two reasons. First, because gyms and fitness centers do go belly-up with some regularity. And one possible motivation for their making such an offer could be that they are experiencing some financial pain and are trying to get some money quickly upfront.
The second is that "stuff happens" and it's not that rare for something to happen to prevent you from going forever--your work schedule changes, or they change their hours, or you experience a minor injury, or you move, or a gym you like better opens up nearby.
Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness; Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.