New Days are a Comin'

Went to my first Tri clinic today and am amazed that when one is given the feedback and proper instruction on things like form and speed...one can do amazing things!
I joined this 8 week training program because this is my first Triathlon and I am a runner. Not a swimmer, not a biker. A runner.
A runner will run, most times, before doing any other form of exercise. At least I do. Maybe that's why I am also becoming more injury-prone?
So- I feel like I can swim now. The bike thing- eh. I need some cushion shorts and maybe I will feel better about that part.

Interesting conversation followed class today. I had asked when I should run my long runs now with the tri clinic being on Sundays. Keep running them Saturdays or run them the same day as the tri clinic?
What is your mileage?
My last long, long run was 16 miles?
What???

So- to make a long conversation shorter I came home with the impression that I should only be running 13-15 miles on my long runs. I have run my last three marathons peaking at 20 miles, then tapering.
I am looking for more information on this, since all the training plans I have seen consist of long runs gradually increasing to 20-22 miles (with recovery weeks splashed here and there) then a taper before the race.

Any thoughts on this one??