The amount of time it takes to recover after running 26.2 miles or more is significant, even if it's a training run and not at fast pace. Keep in mind that you damage your muscles when you train. There are diminishing returns on overloading distance for one run. This is a big reason that coaches have their athletes break up their mid-week runs into 2/day. It's much easier to recover from two 8 mile runs than it is from one 16-miler.
Keep in mind that the above holds true for most of the marathoning population. I don't know you, your history, etc, so I can only give you what I know to be most directionally correct for the masses, including faster runners on this board.
Building a solid marathon runner takes seasons/years, and there are no short cuts. After each season, you should notice your body adapts a little better to longer runs, and longer races. Be patient.