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how about 'legionnaires march'? run 100 yards, walk a 100yards, cover 7.5 miles an hour without getting tired?
[you can change them into metric]

yeah I tend to go into that mode when I can't run any more. If I'm well rested and the body is working well, I should be fine. I just need to get past that first cutoff in under 3 hours.

I think I can do it ok. But it will depend on the weather a lot. I'm finding that I really struggle when it is hot. Which is a big surprise to me.