I have run three marathons, all of them using Marathon Training Plan A from the 2nd edition of the Daniels book, which is the precursor to the 2Q plan.
If those plans parallel each other as closely as I think it does, then you are looking at a 6 week block of work at vo2max pace, then a couple of six week blocks that almost exclusively focus on threshold and marathon pace efforts.
If that is the case, you could easily drop two weeks from that vo2max phase and turn it into a 16 week plan. 4 weeks of vo2max work would be enough time to get the desired benefits of that phase - some vo2max adaptation, and some work at a pace that will make the ensuing 12 weeks at primarily threshold pace feel more manageable.
You might be able to even trim that down to three weeks, but at that point, you will lose some of the vo2max adaptation. That's not the end of the world since vo2max is not a limiting factor in the marathon, but improving your vo2max does make your threshold pace drop some in advance of that big block of threshold work, so there is value there.
So if you wanted to drop down to 16 weeks, drop 2 weeks of vo2max work from the first phase. If you want to get down to 14 weeks, drop 3 of those weeks and one of the weeks from the second phase.
Alternatively, just don't view it as 18 weeks. Break it out in your mind into separate training cycles of 6 weeks each. If I recall correctly, those 6 week blocks have two subcycles within them of roughly 3 weeks each. So treat them like you do the last mile of a 5k - break it down into smaller pieces and just focus on the smaller pieces, not the whole mile.