I am currently doing a walk/run program. I know that I could do much better on my running but I am having a hard time getting over the mental hurdle. I start out well, then I really start to think "Oh my, I'm going to run out of breath or get too tired, etc..." Then by body really starts to tense up and it makes it 10x harder to go any sort of distance.

Any suggestions?

Here is what works for me, LOUD hard rock music, I love it, I try to run to the beat and it works...If I forget my music, I plan my meals, make mental lists, think about anything but running... Sometimes I will count my steps in threes, just repeat 1.2.3.,1.2.3., 1.2.3., or any other mantra you may chose ... When the bugs get in my head and I start to say things like you mentioned to myself, I give myself heck and think hard: "My feet aren't sore, my breath is good, so what the heck is your problem, just keep going..." Oh, just thought of another one my coach/friend showed me, you look down with your eyes almost semi closed, and tell yourself " Relax your face, relax your shoulders, relax your neck, just relax and breath " and keep repeat this for a few rounds, it really really helps...

There are a gazillion head games you can play with yourself ... I'm sure others will come around with some ideas...