In reply to cfer:
> Can I do them on a cx or am I going to get left behind?
Having crossed paths with some sportives I would say nope.
Sportives generally have a wide range of abilities from those who really should be doing proper racing to those out for a laugh. So you will fit in somewhere along the lines. choose one of the right distance and fit into a group of riders with similar pace and you will be sorted.
> Also is it going to be worth joining a club, again I dont want to be left behind or laughed at for having a cx at a road riding club
Impossible to answer. Depends on your personal preference and if you have any mates to ride with. If you want to get serious about the whole racing thing then you will probably need to join a club at some point. Particularly for road racing to learn how to play well in a group.
As for being laughed at and left behind. First probably not but latter quite possible depending on the club and how fit you are currently.
Check what the local clubs offer. Some will just be for very serious riders but others are likely to offer a set of different rides for different ability levels allowing you to progress up.