"The agony of the seat" You need to get your tushie trained. I don't think that anything but riding will accomplish that for you. Think endurance more than strength. I don't think that intense 5 mile ride is going to help you very much. Even that 20 mile ride is probably too short. I'd suggest weekly training rides in the 30 to 50 mile range and this is a case where more really is more.
Other than that, 280 miles in 5 days should be a reasonably achievable goal.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.