I agree its not a good idea to make significant changes once your into a training cycle. My orginal post was looking for subtle issues that I should consider before beginning my already planned interval phase.
By subtle I meant, for example, the relative benefits of various interval and recovery patterns (ie: 800m/800m or 800m/400m, etc) or if my planned interval pace seemed in line with my expected marathon pace.
I was also looking for things like: drop the 800m intervals in favor of 2400m, or drop the 1600m and add some 400m; along with an explaination of why that would be a better idea than what I had planned.
To some degree the 1600m and 800m distances I have choosen for my program are based on my ‘feeling’ that for a marathon runner the longer interval distances would be better. I thought that more experienced runners would be able to confirm that or suggest better speed work than I planned.
With only 7 weeks to go its too late to make major changes. On the other hand subtle changes as you enter each phase may be useful. In 6 weeks I probably will be asking for advice for my taper phase too.
Thanks everyone, all your responses are very interesting and educational.