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I’d head out Hathersage direction. Out through Porter Valley, do a lap of Lady Cannings, Burbage Rocks, round Longshaw estate, towards stony Middleton and pick up a loop round Eyam (I can send a file for that), head back via nice gravel near the glider station above Eyam, bit of road from Grindleford to Hathersage, stop at pool cafe or the deli then roughly back the way you came.
That’s a gravel/ xc option but if you want more gnarly I’d get train towards Edale and Hope valley and ride as much round there as possible.
Feel free to pm for route links?
Say 6-8h riding max, at 15kph average? Max 100km though, with options to shorten if weather/mechanicals and such. Include a lunch stop..