If you're talking about static stretches here (which I assume you are as it looks like you're talking about tight hamstrings), I would be very cautious.
If you're stretching in the park, I'm assuming you're doing it directly before or after a run. Static stretches before running has been shown to reduce performance and possibly increase the risk of injury. The advice is to stretch at a different part of the day, so if you run in the morning, stretch in the afternoon and vice-versa.
Dynamic stretches are a different matter. There is more evidence in favour of them providing some benefit if carried out directly before running. Dynamic stretches are those where you keep moving, rather than holding the stretch for x number of seconds.