Depends how many hills on the ride- how many times you have to slow down or accelerate for corners- how many times you stop for PIE.
In my area with hilly bits- I average 15mph for around 50 to 60 miles. But an organised Metric on an average route and my time is around 4 hours so about 16mph. a full 100miles and I am yet to break 7 hours which is 14mph.
Solo riding and organised events do lead to different ride speeds for me. The organised rides are always just a bit faster.
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How long was I in the army? Five foot seven.
Spike Milligan