It is definitely all mental. As long as you have a rhythmic breathing and steady pace, you can go as long as your brain will let you. I struggled yesterday (Week 5 day 1) and there were at least 3 times during that where I had the thought of "I'm not going to be able to finish this one without a break. I'm going to have to stop and rest. This is the one that ruins me." And even though it was absolute agony that last stretch, I kept saying "only a minute and a half left... only a minute left... only 30 seconds left..." and while most people will start to feel more tired while thinking "oh gosh there's an entire minute left!" I find it helps push me on. Because as I'm running, I remember that I CAN run 5 minutes. I CAN run more than that if I want to, and usually those thoughts of "I have to stop" are during the last 5 minute run. So I just look back at not even 5 minutes earlier; I have run 5 minutes without stopping TWICE now; I can do it a third time. If I can run 5 minutes twice and I made it this far in the last stretch, this last minute and a half isn't going to be the end of me.
After that run and I dropped down to a walk and caught my breath, I left the run feeling the same as I always do after a run - refreshed and awfully tired. That's how I know it's not too much - I didn't collapse, I wasn't limping, or curled up in a fetal position cradling a torn muscle, and I wasn't super dizzy. I did it.
Tomorrow is another story, but is the same idea. I'll get to the 5 minute mark and see that there are three minutes left and I'll groan in agony because that's THREE MINUTES. My legs will be tired and burning, I'll be breathing hard, but I know I can do it. I'm going to at least try anyway.
I can't remember their name but one of the other posters before me that suggested going "okay 30 more seconds" after each "minute milestone" is absolutely right. It's up to you whether you want to give in to your fatigue or not, and sometimes it's just the thought of "I can't do this" that stops a lot of us FROM doing it. For me, unless I feel like there's something SERIOUSLY wrong (torn muscle, on the verge of passing out, etc) there's no reason for me to stop. My condition is the same it's always been, I just have to push through it and complete it. I'm breathing hard but steady, I can slow my pace if I need to, but one way or another I will run the entire duration.
As for Friday, well, we'll see when we get there >_>