As a back of the pack ultra-runner, I find that I end up hiking. A lot. It's a very different discipline from road running, in that planned walking is normal and expected.
So, as well as C25K, I'd suggest simply packing a sandwich and heading out on the hills for the day, really as often as your schedule and the weather allows. IMHO, for anyone simply aiming to complete, that's as good training as running.
I'd suggest checking out XNRG events rather than Centurion. That's because Centurion aim their events at runners and have time cutoffs. If you're not fast enough, you'll be pulled from the event. XNRG are hugely supportive to newcomers, and will wait til you've finished, no matter how slow you are, which removes one stressor on the day. I'd also look at LDWA events for the same reason.
Ohh, and the kind of pack that's comfortable carrying large weights whilst walking for hours is different to what you'd pick for running. I can't run more than a few yards comfortably in my camino pack - the movement of the frame is all wrong. Which is another reason to train to trek, not to run or cycle.
I walked the Camino to Santiago last year, so 500 miles with a pack.
I was surprised how little running fitness translated into walking fitness.
I was suprised how much a difference a relatively light (8kish) pack made to how tired I was.
I was suprised by the cumulative weariness of walking day after day after day, with no rest.
So I'd recommend hillwalking, increasing your packweight til over what you expect to carry. Learn how to adjust your pack - tiny differences in strap length make a huge difference in comfort. And lots of back to backs, because you won't get rest days once you're out there.