Eat to Train or Train to Eat?

They say when you are training for something like a marathon or a triathlon that you shouldn't change anything about your diet in the days leading up to the event. I can't help but wonder though: what if you've been eating nothing but junk? I thought I'd better play it safe and eat equal parts unhealthy and healthy food today in preparation for my sprint distance tri in the morning. On the lunch menu:

Let me just say that this ice-cream is SO good. All of the PC Ice Cream Shop Flavours that I've tried are. Something tells me that they are seasonal. I hope not.