First of all, no you don't have to join a gym to successfully exercise aerobically. Anything that gets your heartrate up, above resting level, for a sustained period of time without breaks is considered cardiovascular work. Mowing the yard, walking the dog or doing housework can be aerobic. Machines, such as what you'll find at the gym, allow for monitoring of your intensity level. This is important as it pertains to improving upon your level of fitness. In other words, if you think that you're getting enough cardio from your job because it requires you to stand on your feet all day and you do a lot of moving around, think again. Unless you're consistently pushing yourself above your current level of adaptation, then you're not improving upon your level of fitness.