I've completed my 3 runs of week 3. All in the snow and cold.
I'm now debating whether or not to move on to week 4. Week 3 has been much more of a challenge and it does seem like a bigger jump between weeks 3 and 4 than previous weeks, but on the other hand, I've found the second half of the run has been easier than the first, almost as if my body is protesting at first but then settles down, so I'm wondering if the longer run won't actually be as hard as it sounds.
At the moment, I think I'm going to try week 4. I'm terribly impatient and I want to run for 20 minutes NOW not in a couple of weeks time! If it all goes horribly wrong I can always go back to week 3 for a couple of extra runs.