You can change the difficulty of the game from the options on the main menu. There is only easy and hard and the only difference I could tell is that your awesomeness meter depletes faster and you don't have as much time to perform the "actions". Other than that it is a breeze to play through.
You can tell which levels you have completed on hard by going into the mission select screen, choosing a story and then selecting a level. If you see a gold star on the top left of the level icon then you completed that level on hard. If you do everything else first, the run through on hard only takes a little under 2 hours if you skip all the scenes.
If you want to knock this out in one playthrough, exit the game on level 1 and change the difficulty right away. This will make getting the action achievements a little harder though, but only marginally. To do this, hold your left arm out at a 45 degree angle and pause the game. Select Exit Level and choose "Yes" to return to the Lair. Go into options and change the difficulty to Hard and return to the menu. Now go into mission select and select the first level and you're good to go.