Speed? I have only ran part of the C2C last August when I completed a shortened route in 3 day 15 hours all other trips have been pure walking. I just go at what feels my natural pace without rushing, but I don't linger or make many stops maybe a couple each day. I get the miles in because I have good endurance and walk and walk from before sunrise (My favourite walking time)to Dusk (My second favourite walking time).
I have done many walks previously in' normal' time, e.g:-
West Highland Way 95 miles - 7 days.
Dales Way 84 miles - 8 days.
Offas Dyke 176 miles - 11 days.
I don't like leaving my wife for too long due to her health issues, so I try and spend as little time as possible on trips, plus I just love high daily mileage - I'm an addict!