Isn't zone 3 basically a race pace? Seems like you have to train in it occasionally if you're a racer, but like already said, if the fatigue is accumulating from too much time in zone 3 then you'll just have to back off, but I know that's easier said than done. I'm not a racer, but I am a life-long commuter that needs to get places in a fairly quick time, so I do spend a lot of time in zone 3.
My recommendation is to get comfortable in all zones, but have the smarts to know when to take it easy...again easier said than done -- I wish my body could handle what my brain/ego wants to do
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