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https://www.trainingpeaks.com/blog/shoulÂd-i-run-long-runs-at-race-pace/
Yeah this makes sense and is valid, but the half marathon plan referenced has long runs up to about 20km (at 'long run'/easy pace), but then running at race pace is only ever done up to a total of 8km in a session. As @yoshy said, I am struggling to see how running 8km at pace would be sufficient preparation to then go and run 21km at that same pace.
This is about Marathon training, but outlines the principle I think?
https://www.trainingpeaks.com/blog/should-i-run-long-runs-at-race-pace/