Ray,
A few thoughts:
1. TP does not let us set a setup zone for “Marathon Pace” or any distance pace. We are limited to setting Zone 1, 2, 3… each as a % of threshold pace. Therefore, your individual marathon pace may vary from the set Zone X training pace. That part is normal. For some, Marathon pace is Zone 2, so they would also ignore the Zone X range and use their own pace as well.
2. If your HM pace is 5:54, I’d put your Marathon at 6:20, not 6:15. 6:20 falls directly into your Zone X. But, that is assuming your running the Marathon maybe 6 weeks after the HM. If you’ve trained for 12+ weeks since your HM, I’d expect you to gain new fitness, and your actual marathon pace could be <6:15.
3. Just because you can run 50 minutes at 6:00 doesn't mean you can run 3 hours at 6:00. You know this, just clarifying for other readers.
4. The purpose of this workout is not to run the 50 minutes as fast as possible, it's to get to feel exactly what marathon pace will feel like. Run the next one at what you really think you'll maintain for the marathon.
5. Yes, your current zones could be a bit low? But, again, go back to response #1 above. Don't put too much stock into Zone X as the perfect marathon pace, you could pull off a low Zone 3.
David