AS far as stretching goes, I know that gymnists and ballet dancers all hold their stretch positions for about 5 minutes each, slowly increasing the stretch the whole time. They also stetch to the point of little pain. And these are some of the most flexible people out there.
You really must regualte your breathing when stetching. I know there are certain positions that restrict the breathing, but as a whole... breathing really coincides with the pain. If you regulate your breathing to almost a meditative state, there is no pain because your blood is flowing freely throughout your body.
Plus this type of pain stetching has more memory to it. Your body will stay flexible longer than taking it easy.
For example, there is a 63yr old male student whom never was able to touch his toes or come anywhere near the splits. Some of my students who are 35, 45, and 60 all complain about their being old, and telling me I don't know how it is. Well, I'm a young teacher, so maybe I don't, but I always tell them they are as old as they think they are. Anyways, this 63yr old student... let's just call him Mack, asked me how important stretching was to TKD, realizing that his flexibility was hindering his technique potential. I told him that TKD is done best with Maximum flexibilty. I always help my students out when stretching, and I would always take it easy on Mack, until he told me he wanted to touch the floor w/ his splits no matter what. Big surprise to me, I thought he was nuts! I didn't want him to break a hip or anything, so I refused at first. Then he explained his situation, and told me how bad he wanted it. So next time we did the splits in class, he asked me to push him all the way to the floor... and I did, a loud pop sound came from his joints, and he was in the splits and couldn't get out. We helped Mack up, he had to sit out and watch class as he could no longer participate. About 3 weeks went by, and he was taking it easy on stretching, doing what little he could, and had went to the doctor to make sure nothing was broken. Towards the end of the 3rd week, he finally felt that he was able to fully stretch, and low and behold, this 63 yr old man, who has never done the splits in his lifetime, now had perfect splits!!! And he's had them ever since!!! He has the nicest vertical side kick! I always use him as an example to all my students whom take stretching lightly. I feel, if Mack can gain perfect splits from almost nothing AT HIS AGE... then anybody can.
It all depends on how bad you want flexibility, and how much youa re willing to put into it. Now I'm not going to say I DON"T recommend doing something like this, because obviously it worked for Mack... but its all about personal preference and perserverance.
I just thought I'd share my story with you guys. I hope I've inspired some of you to put more effort into stretching, its very important and can prevent injury. I'm all for it.... Good Luck to everyone.