Wow, half an hour warm up? That seems a little long to me, I would only usually spend 10-15 minutes most.
I think there could be a few breathing techniques that could help you out, most of them are slightly yoga-like. Practising them can do you the world of good, I think there should be some articles lurking around the training section.
Walking isn't cheating! I've been running for 3 years and there are still runs where I feel like I've been hit by a truck and need a quick breather, I wouldn't worry too much about it.
A lot of the time walking is partly willpower, it can be hard to push through the moments where you just feel tired with heavy legs. It gets better with practice, however if you feel nagging pains its always best to stop. 