Hi Steve, first of all well done for getting your times down so rapidly.
Most clubs will have one or two training sessions a week. Apart from that tho' the main benefits of a running club are that you have other people to train with, a wealth of experience re injuries/training etc, a good social scene, cheaper race entry fees. The first few times you go along they'll see what your pace is and fit you in with one of the established groups.
Although you're obviously doing well on your own I'm sure you'd benefit from joining a club esp when, as is bound to happen, you go through a tough spell.
Keep on running! ;o)