Free beginner IM plan

I assume you read this, which answers most of your questions.  The plan is meant to help you finish your first IM.  It assumes you have been training consistently in the 8-10 hr/wk range for some time (several months) before starting it.  With that as a backdrop, a 90 min long run in week 1 should not be unexpected nor overly challenging.  If you aren't at that level, then the plan may be too aggressive for you. 

I do not see the back-to-back-to-back runs the next week.  But, I would encourage you to think about using frequency in your running if it is a weakness.  That is, especially if you feel you must cut back the duration of some runs, add in some other short runs to keep your volume up.  There are some good threads around that discuss why this is a good idea for many people.  Usually the 'I can't recover' excuse is a sign that the individual simply doesn't run enough.  And more frequent, shorter, easy running is generally the best way to build the durability you'll need for IM.  It seems somewhat counter-intuitive to many who think they need more time off in order to recover, but works for the vast majority.  However, building that into a 20-week plan may be somewhat aggressive in itself (it does take a bit to build that frequency into your training).