I have not yet had a chance to read Brian's E-Plans. But there are a few rules that I live by.
1. Always practice at a level that requires 100% focus. You can do this by pushing distance, speed, stroke count, etc. You want to swim at a distance that you sometimes add a stroke or lose focus, but can regain it if you refocus. If your race is sooner than the plan allows, I would switch some of the tempo based practices to distance based practices.
2. Remember that you can always roll onto your back and chill for a while in a race if you need to pause and refocus. You will do better swimming your race in 100 stroke intervals with good focus and pauses for rest than if you tried to force your way through the distance.
3. Make sure open water practice is a regular part of your practice. OW is different enough that there is no substitute for practicing in OW. My favorite example of this was my own first open water race. I was a 20min miler when I was racing regularly. My first OW race I lead my entire wave about 200yds off course and added 5 minutes to my race because I had not practiced sighting well.
I hope this helps.