I know this because I'm a graduate who had to go right back to W3 after injury. I find the short run-walk training far more exhausting and difficult than running continuously for 45 mins, which I was doing comfortably before injury.
So this is a message of encouragement to newbies: try not to think thoughts like:
"This is so hard, I'll never get through it."
"I can't run for x mins - I'll die."
I still think these thoughts too as I progress on to W5, partly because I've lost a lot of fitness, but know this: This is the hardest it's ever going to be.
As you go through the weeks, it will get easier, although W5 R3 was hard first time round, probably psychologically more than physically. But you will get through it!
Also, if you're running outside and struggling, switch to the treadmill - it's a million times easier, giving you an important psychological boost to get you through those tough weeks. There'll be plenty of time for outdoor running after graduation when you're bored silly by treadmills!
I was on the treadmill yesterday for W4 R3. Aaahh... it was almost like being sat in a comfy armchair by comparison!