Competitive running clubs Herts

I have got really into running recently, I got into it as a boxer who purely did road work for fitness and making weight. I was always a good runner and since I've given up fighting I have focused more on running, and have got really into it. I ran my second half marathon in Bristol on Sunday in 82.40 and it's got me thinking i'd really like to join a running club, but without wanting to sound like a total snob, i get the impression from a lot of clubs that they're quite casual and a bunch of people who go for jogs together.

Can anyone advise on the difference between casual clubs and competitive running clubs? And can anyone advise on any in the Hemel Hempstead/ Hertfordshire area that are worth checking out?

Thanks a lot.

I don't know if there are any closer by, but St Albans Striders have some pretty handy runners.

For a bit of background research, have a look at some local race results and see which clubs are represented at the top end.  Power of 10 is useful for that sort of thing.