Week 16 & 17 Long Runs
I asked Coach P today about these runs, and thought I'd share what I learned. First of all READ CAREFULLY the instructions for week 17! 
Although the time is listed as 180 minutes, the text says "3 hours or 15-18 miles, whichever comes first". Now, that 15-18 mile requirement is pretty squishy - could be a 30 minute or more difference. What I would say is if you're a faster runner (Vdot above 47 or so), just go 18 miles at the prescribed effort levels, and STOP. If you're in the 8:40-10:30 LRP range, again, don't go over 18 miles, but you might find yourself getting near 3 hours. And if your VDOT is getting you in the 12-14 min/mile range, DO NOT UNDER ANY CIRCUMSTANCES go more than 3 hours, even if that ends up being less than 15 miles. If this concerns you as, say a first time IMer, never having done a marathon, worried about race day when you may be running for 5 hours or more, and not being ready, read this wiki post, and the firm thread linked in the first paragraph:
http://members.endurancenation.us/R...1+or+LOWER
Now, if you're like me, and you hate to run for long periods (for me, that's anything over about 15 miles, which can take me 2:05-15), Coach P said it's OK to "split" the run. His recommended method: Split the time 60/40 more or less. For example, a 150 minute run becomes 90 minutes in the morning, 60 minutes in the evening. Again, DONT RUN MORE THAN 18 miles. Now, as to pacing: morning, 30 minutes @ LRP, 60 minutes @ MP. Evening, probably LRP to a bit faster. EG, my LRP is 9:07, I probably want to start out there, and not go any faster than 8:40-50. Be happy with anything LRP or a little faster. And for the week 17 run, I'll be doing probably 12 miles in the morning, 6 in the evening.
He said splitting should only be considered in three circumstances. First, your schedule (work, kids, whatever) does not allow time for a run that long. Second, Coach sez it's better to run at the faster paces and split the run, than slog along for 2.5-3 hours at nothing faster than your LRP (or worse). OR, you have trouble recovering after runs that long hitting your subsequent workouts. Those last two are me.
Be aware that the second run in the split system is brutal, not a cakewalk. Your thighs will hurt, your mind will rebel. But pushing through is not only good physical, but also mental training.
Comments
Yes, this is new this year. Freaked me out when I first saw "180 minutes", as I REALLY don't like running more than about 17.5-18 miles, which takes me 2.5 hours. It was added, I think, in response to concerns from/about runners with lower VDOTs, who may not have been getting the mileage and time on their feet they need if they are running (really running) 5+ hour marathons on race day - the 12 minute RockStars.