Ah, I remember this feeling well, heading into my first half-marathon. Little bits of doubt start to creep in and the danger of burnout is lurking overhead. My first half was a toughie - hot, long, and a jolt to my confidence. The second one I OWNED. My goal in this triathlon is to actually enjoy the first one!!
What I am having to remind myself of: stay relaxed and confident, keep the focus, take it one day at a time, and maintain the longer term perspective that this is only the first of many triathlons so the primary goal is to run a confident race within my capabilities and enjoy the experience. I'm doing the homework and will be well prepared for a rookie race.