Give it a go, see what you can do, and then evaluate from there.
I am only on my second week at the moment, and when I started last week, I couldn't complete all of the runs until W1D3, when I got through every running part
. And you can always re-do weeks. My plan is if I struggle to complete a stretch, to keep doing this week/day until I can do it.
The trick, I've found, is to go slow. At the moment my aim isn't distance, it's time. So when I'm running, it's more of a slow, bouncing on the spot kind of jog (lol).
You could start off just walking for 20 minutes on a flat surface, and build up to the C25K. I started walking at the beginning of this year, for a distance of 5K, and at first I found that that was enough for me, but now it isn't. So I'm now trying C25K. So far my un-athletic body is really enjoying it.
I hope that helps =)