As far as a "practice marathon"; I found that they pretty much beat me up if I do them sooner than 8-10 weeks before my goal.
As far as the goal marathon itself, remember this: it is much easier to pick up the pace after too slow of a start than it is to keep from crashing after too fast of a start.
In terms of pace, there are too many variables to say "run the first 10 miles in 1:27:34", etc.
Just make sure that, after those first 3 miles pass and you are getting warmed up, you keep things under control. If you are not getting impatient with the pace after the first 10 miles or so, you probably started too hard.
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