Depending on what your 5K and 10K times are, I don't think that 4 hours is all that aggressive of a goal. One thumb rule: 5 times your 10k time is a good marathon goal (about what I do these days).
You seem to have a good base to start training; ramping up to 45-50 mpw shouldn't be that tough (if you do the increases slowly enough)
About speedwork: I think that it helps. I like to do a midweek speed session (either hills, tempo, or some longer intervals (800-1200 m)) and a weekend race.
I am a "high-3:3X-mid-3:4X" type of guy. Not fast, but I want to do something other than shuffle at the end of the race. :-)
Good luck, and let us know how it goes!
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