make sure you are breathing with your diaphragm (eg makes your stomach move not your chest) and as mentioned dont run too fast to start. having said that it takes me 3 miles sometimes before my breathing catches up! it may be that your runs are not long enough yet for your breathing to settle and your 'second wind' to come in.
I have the salamon Xt wings 10+3 and have used it in a half and in a marathon (the half was to try it out and the marathon was an offroad one) I found it to be pretty comfortable for the half but but for the marathon with a full litre water bladder in I found that the bladder dragged down the back of the pack inside and it meant the pack rubbed a little bit where it folded. However i was using a short bladder that didnt touch the bottom of the pack so this may have been the problem. it is more comfortable than a rucksack or the camelpak i have but also where you have the extra fabric across your chest it does get a bit warm.