Well can chalk it up to one of three-ish things.
- You had a great 70.3 before hand which is a positive, so you could say the plan was working and you had a bad day when it came to the 140.6. That happens, just sucks when your off day falls on your big race day. Bummer.
- You had a great 70.3 before hand which is a positive, but you just couldn’t recover enough from that to be ready for a 140.6 a few weeks later. You said you smashed your 70.3 PR, people tend to forget that will take a lot out of a person and they might not be ready for another peak performance.
- The TR plan for the full was not ideal for you, which happens (but it sounds like a lot of it workout for you based on the 70.3 PR). A canned plan is not always going to be best for someone. If you did really well with a personal coach in the past that might be good to look into again.