I did a lot of running to shed a lot of body fat, usually half hour or more runs on the treadmill, at first I started to get stiches after a couple of minutes and had to stop. However I was determined to keep going, so each time I would go in I'd aim for 5 minutes then say 7 minutes then kept going up and eventually I eradicated the problem of a stitch. Imo it's all to do with the body really, if it's not used to that type of exercise motion then problems occur but if you get used to it there shouldn't be a problem.
Another thing I found was drinking water, I found that if I went on a treadmill and didn't drink water within the 30 minutes, sure I'd get thirsty but I would not be interupting breathing patterns, thats another issue is the breathing, tell her to breath through her nose and exhale through her mouth, variety of ways you can do it and theres pleanty of articles on the net with different examples:
http://www.runnersworld.co.uk/news/article.asp?UAN=24
And thats also another thing to add to my first point, getting used to it. If your girlfriend has not really been a runner or someone who is quite fit on the exercise point of view then it does take a while to get into it, but she will get there
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