Everything.

Before I joined my running group I weighed 225 pounds and was a 9-minute-miler who ran roughly 15 miles a week.

Now I weigh 184, can crank out sub-6-minute miles, run 50-70 miles a week, and have qualified for and run the Boston Marathon.

The biggest keys are just having people with experience who share their wisdom, and having the peer pressure to actually show up for a run. Also, I've made the best friends of my life, all in less than 3 years.

Good luck with your club! I usually run solo but have joined a few clubs. One is a bit too far from me and then the local one that just started is so new and so small that they don't have a variety of runners yet. I went with them on a trail run and they all ran at a pace that was faster than my road pace! Come to find out they all run at paces that are considerably faster than mine. I haven't run with them since.

I know of larger, more established running clubs that have people at all levels. I'm waiting for my local running club to grow so I can easily run with people at my level.

I saw a local running club online but it was difficult to tell how active it is. The one member who organizes everything is way faster than I will ever be, so I wonder why I should bother going to the runs.

I think I would be the slowest runner there--so would end up running by myself. Which is not the point of a club.
Still thinking about it...

I got a cool singlet and free admittance into races they sponsored and a discount at a local running store. And I attended one really awkward Christmas party.

I ended up quitting because I was obligated to volunteer at one race a year and I could never get on with any of the cool races like the trail ultras - instead I'd be stuck working at little races where the roads weren't closed and angry drivers would scream at me and try to run me down with their minivans and not respect the authority of the orange vest and flag. F**kers.

I'm in two clubs:

Club Northwest. This group has formal training runs with people who are much faster than I so I don't usually participate. They sponsor a couple of larger fun runs around Seattle, several run series, and also have a Masters meet each year. I mostly maintain membership for race discounts and their magazine with local running events. I also get a 10% discount with several local running shops for being a member. One of the cooler things that they do is have local track meets during the summer for all ages. I took my boys last summer and they participated in a lot of events and got very interested in running. Cost: $75 per year for me and my kids.

Richmond Beach Running Club: I wish I would have found them sooner! I remember when training for my marathon last winter I would see them running and wonder who they are. My son and I joined a couple months ago. We meet every Saturday and do runs together - there are people of all paces. There's a Sunday run too. There are also weekly clinic series several times per year. Sometimes i can't go because I have other events and my 10 year old son will go by himself and someone always runs with him! A few weeks ago he went almost 9 miles with another member. This is the most friendly, supportive group of people I have ever met. Cost: $0 (though I'd gladly pay)

In my last club (Black Hills Runners), I was the "expert," so go figure haha. I just joined the Ohio River Road Runners Club and will get back to you. Seems like they have a lot better membership and more "options" than my last club, which was mostly old folks who got together to shoot the breeze and occasionally run.