I've already completed my first 20 mile long run of the year, and will do my 2nd on saturday. All my training long runs will be much lower than my goal pace, my first 20 miler was a minute and a half slower per mile. This is because I don't want to burn out by training too hard (which I sort of did last year), and my aim of these runs is to boost endurance and train my body to take fat for fuel efficiently.
However, I will run the 20 mile warm up race at my goal pace for practice because i will have already completed 4 20 milers beforehand.