I've done IMW 3x plus the LCW and various training rides and been fine with a compact and a 12-28 cassette. Just soon up the hills, don't start them with a defeatists attitude, they are all rideable on both laps.
Wales is good, as you can train on the course relatively easily. There's the Long Course Weekend, so you get to do the course (swim and bike) in advance. It's late in the year, so a summer of king rides and runs and build events in generally good weather. Support is awesome, and it has a well deserved reputation of being hard.
However, it ring have a 'destination' feel, and you'll get s slow time.