It is called "oxygen debt" and this graph explains it all. Basically - when you start exercising , you don't have enough oxygen in your blood to feed the muscles that you have started to use - so the heart and lungs have to race to catch up. After a while (but quite a long while maybe 10 minutes) you start to settle down and catch your wind (often called 'second wind" ) - trouble is C25Kers are not running for very long so often they feel it harder - the secret is for those who reach the latter stages of the programme where they are running for longer times - you need to "warm up" for longer than 5 minutes and much more intense than that brisk walk you have been doing!! 
Walk for 5 minutes , but then jog a little for a few minutes and do a few pushups and squats - anything to get your heart rate up before you start your C25K task
Something really important to know about running! - Couch to 5K
Something really important to know about running!
That makes a lot of sense Bazza, Thanks for that.
Good information Bazza thank you!😀😀
Thanks for posting...
I now feel justified in my longggggggg warm-ups. Thanks for that Baz 👍
Yep, takes me 20 minutes running to be able to breathe, no matter how slowly I start off.
Try doing some energetic exercise before you start to run - get that heart and lungs pumping ( without hurting yourself) - do some pushups - you will find that the initial stages of your runs are not so painful. Some time ago I was watching the elite competitors on the front line of a large half marathon - although they had to run 21.1 klms, these runners were doing 100 metre fast sprints away from and back to the start line BEFORE the start. Couldn't understand why they would be doing this - but I know now!!
I’m afraid a lot of exercises like squats, pushups etc are a no-no because of my injuries. Scoliosis, fractured femur in the past, badly mangled knee. When I used the gym, there were very few activities that were recommended for me.
Actually - the problem for us is not the oxygen debt, it is the oxygen deficit at the beginning which leads to subsequent oxygen debt.
I wish I'd known about this years ago. Since 1991 I've never lived l more than five minutes away from a park, but when I tried to run in the 1990s I just always gave up because getting going was too hard and I felt self-conscious about running such a short distance and stopping.
If only we'd had C25K and this forum back then, I could be celebrating a quarter century of running.
Then let's get you on the road towards celebrating the next quarter-century of running instead! Definitely better late than never.
Brilliant explanation. It is why no-one should fear W5R3.
Curiously, I quite like this part of the run as I feel my heart and lungs working to prepare me for the run ahead.
It was an epiphany for me when someone told me about this. I was able to lose the spirit of panic/despair that previously gripped me during the first 10 minutes; I know the feeling won't last.
And you're right, the trouble when you start is that all your runs are within that oxygen-deficient period.
Take heart, C25k beginners, once you're running a bit longer, it'll actually get easier.
Very helpful Bazza :), thanks for posting
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