yOUR GETTING slower??!! put on the brakes!!! NOW!!
Ok, if you want to enjoy that half IM then you need to step it back NOW! If you don't want to deal with the coachs ego right now, then just tell him you did the distance or whatever. I dunno?? I hate to advocate lying. But if you don't want the stress at this moment, just do what ya gotta do!
Yes the heat will slow you A BIT. But I'm training in Phoenix Monsoon season, and my average hasn't dropped like that.
You are breaking down your muscle fibers without giving them a chance to heal and get stronger. As said, you will eventually get sick and or injured. And my guess is it will happen sooner than later if you don't rest. For the next couple of weeks I'd actually schedule TWO off days just to get your body healed. But you always need at least one. AT LEAST one!
Get those vitamins in too! Your body needs them right now.