I tend to hold my diaphraghm tight when I run too...I think it's natural...an attempt to protect yourself. But as you get better, and more relaxed, you will take bigger breaths. Sometimes I will do big exhales just to force the air out and makes myself have to take a deeper inhale.
Do you do intervals on the bike at all to help with power?
Want motivation? Get a skinsuit...THAT puts the spark in ya! Sheesh! Who talked me into getting a *@$*@!! skinsuit????