Your gola of 3 1/2 hours may be somewhat high, especially never having done this before. As new as you are to this though, having an 8 mile run under for belt is pretty good, though. Most important are doing 1 long run a week- any more long runs than that you can really risk breaking down physically. You also want to run once a week and your anticipated race pace. Probably you want to increase both these distances 1 mile a week until the long slow run is 20 or 22 miles, and the race pace run is maybe 18 or so. The odd thing about the marathon is most runners don't run the complete distance during training. If your training maxed out at 20 miles, at that point in the race, you still have a full 10K to run, and that can be intimidating, so I would advise to maybe ease up on your goal. Take an easy week every 3 weeks or so and at least 1 day off a week. Rest is a key factor. When you get the long runs stretched out to near 20, you might want to either plan a run so you can stop off at home for a banana or something, or get a few gel packs- they are convenient- you just rip the top off and suck the goo out and take a drink- no reason to stop, really. Good luck!
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