If you try to single out the more experienced runners, they're probably not huddled around talking in groups. You'll more than likely have to move to the perimeter of the race location to find them. Have you ever pulled up to a local race and see a lean runner-type with a race bib on running on the street about a half mile from the start and think you've missed the race?! That's probably an experienced runner warming up for the race.
Experienced runners have learned that the best way to "stretch" or warm up prior to a run is by moving. This is often called dynamic stretching. It's not stretching in the traditional sense. It actually refers to stretching your muscles by using movements similar to running. Simply walking or jogging can accomplish this. Other movements might be high knee lifts, butt kicks, skipping, side shuffles, etc.
Beginning a regular run by just taking off at a slow easy pace is actually "stretching" or warming up your body and getting it ready for your harder pace later in the run. For a race (like a 5K) when you may be blasting off at a good clip at the sound of the gun may require you to do more of the high knee lifts, butt kicks, skipping, side shuffles, etc. before your race to ensure you're good and warmed up for your sprint.
If you dig through the research, there really isn't any proof that stretching is beneficial prior to running, but then again, there is no research that says it's not beneficial either.
There are a couple of "rules of thumb." For example it's best not to do traditional stretch-n-hold stretches on cold muscles. Now I have a 12-year old beagle and the first thing he does when he wakes up is do a big ole stretch and I've never heard him complain of pulled muscles, but most experts will tell you that it's best to stretch a warm muscle because you're less likely to cause injury. Think of a piece of taffy that's been in the fridge. If you try to stretch it while it's cold, it's probably going to break. If you let it warm up to room temp, then you'll be able to stretch it without it breaking. Same principle with your muscles.
Actually what happens when you warm up through active movement is that the viscosity of your body fluids is reduced and that increases the "stretchiness" of your muscles and connective tissue.
So, before a run "stretch" or warm up by moving—walking, jogging, skipping, butt kicks, etc. Save the traditional stretch-and-hold stretches for after your run.
There are times, however, when it's best not to stretch at all. For example, if you've have a sprain or muscle strain with bruising, stretching is not going to help. It may make things worse. Also, any time you feel pain when stretching, you should stop. Don't push through the pain.
Doing static stretches immediately after a really long run, isn't a great idea either. After a long run of 15 miles or more, let your heart rate get back down to normal by walking about 5-8 minutes afterward. Be sure to rehydrate during the walk. It's best to rehydrate after a long run with a sports drink containing simple carbs. This will be the quickest way to replace those carbs needed to repair those muscles you've just worked out on the run. After your walk when you heart rate is back to normal, you can begin doing some easy, gentle stretches. Be sure not to bounce through the stretches.
So remember to "warm-up" prior to your run and save the stretches for after the run.