Why is this? Many of the groups which runners start out training with, make a lot of stops throughout the run to regroup or drink water. The result being that runners are only running 2 to 3 kilometers at a time before taking a break. Now this practice in itself is not a bad thing, these stops are an important part of the camaraderie of group runs and what makes newcomers feel welcome and keep coming back. Start together, finish together.
The challenge is that you are only conditioning your body and especially your brain's fatigue control center to run a few kilometers at a time. As soon as things get uncomfortable, you know a break is coming so you never have to push through. This becomes challenging when you now take part in a half-marathon or marathon and you want to run a good time or make a qualifying cut-off and you need to now suddenly go places in your body and mind that you have not been before.
I got to experience this first hand recently when I became the subject of my own accidental experiment. I try to take my dogs along on as many of my runs as possible. At one point, my female jack russell developed a strange fear which caused her to freeze regularly on our runs. This meant that I was regularly stopping and going back to her on runs. Wanting to help her overcome her issues rather than just leave her at home, I decided to go Dr. Doolittle 101 and patiently train her out of her fears. The result, my runs became run a little, stop, run a little, stop.
Not surprisingly, I was slowly de-conditioning myself out of being able to run long and easy without stops. (Law of don't use it loose it!)
It was not long before I started noticing that on long runs on my own, I did not get very far before wanting to have a break and long non-stop runs were suddenly both physically and mentally very tough even though my general weekly mileage had stayed the same.
This proved once again that What you train, is what you get!
Does this mean you need to ditch your favorite group runs? Certainly not. The solution is to know the purpose of each run and to include a mix of social easy runs and tougher sustained runs in your training week.
Onwards and upwards!
Professional running coach
Cape Town, South Africa