Being social enough to benefit from this idea but not social enough to enjoy making smalltalk with a bunch of strangers, I never liked the club idea. For those who are less social like me, competitive amateur sports leagues that are segregated by ability and age are a nice alternative. Typically, facility access requires games to be strictly scheduled, ensuring that people play instead of chatting. The competitive aspect ensures a good workout, and there are options to play as an individual (singles tennis and racquetball) or as part of a team (softball, soccer, basketball.)
Team sport leagues usually provide some way of signing up as an individual. Either they'll place individuals with teams who need players, or they'll create "house" teams made entirely of people who signed up as individuals. You might be embarrassingly bad, but you probably won't be the worst. If you're playing in the lowest skill level for your age, nobody can really complain -- if they want better teammates, they should "level up" and play in a higher category.
If you're in Boston and thinking about starting to run, I've had a great time running with The Most Informal Running Club Ever on Saturday mornings -- http://informalrunning.com . Nothing beats getting together with 30-60 new and old friends, running by the Charles at a distance and pace of your choosing, and then hanging out with them all for a potluck breakfast.
Our culture for some reason thinks exercise is prescription medicine that should be taken in secret. It turns out that exercise is just like food -- best enjoyed in a convivial group.