try putting some 600/700 metres reps in, do sets of 3/4 to start with and then build up to 6 reps. It will condition your body to run at threshold pace for longer, it will also you to run quicker when you are not pushing yourself. worked for me.
you need to be able run in a variety of conditions, in this country a race may be held in 25C heat, the next year the same race at the same time of year may be windy, raining and is nearly freezing. Eight miles also shows any chafing issues particular in raining conditions.
enough said, you went to the gym but next time go out and get use to rain