I suspect a lot of it is mental, in that the previous continuous runs (w5 r3 and w6 r3) were a novelty and filled with a euphoric 'omg, I can't believe I'm doing this' where as all three runs this week were physically exhausting but also, just like the one previous.
That being said, today (w7 r3) I kept going until I hit 28 minutes and found the run overall slightly easier than the two prior, so maybe I'm slowly settling into it.