Time Trial / Time Trialling / TT

I'm doing a 10 tonight to hopefully shake down the bike one more time and to get a measure of what my threshold power is, and plan to ride at 85% for the first 70 miles and see where that gets me.
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I'd be tempted to go in lower, power wise, and turn it up. Rather than go in high and then blow.

That way you'll finish, and hopefully enjoy it. Rather than something going wrong.

Next time you'll know what is more suited to you.

85% isn't a bad place to start. But It's hard. And I'd guess it's not going to be a 3h40-50 effort.