I do not think your situation and his situation are similar. I do not think you can extrapolate in a linear manner what his training pace would indicate versus what your training pace proved out.
My reasoning is that in all likelihood someone that is running 6:17 for a marathon is very well trained and has been for sometime. You don't need to hammer long runs because you know that's not the key. I bet you have other workouts that are much faster.
The case of the 18 miler at sub 8 pace for Mr Rossi (with walk breaks) seems to be a case of "humble bragging." The training run was not necessarily a part of a long term training plan other than "run a long run."
Another thing that I will probably fail to communicate properly, is the idea that there becomes a point at which a pace does not feel like running, or at least like "running right." I think this leads to less trained people have a much narrower range when it comes to race pace and training pace.
I bet you had an easier time of running 7:15 pace than he did at 7:54 too.