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Generally your long run is run slower that target pace because you are already putting a lot of training load on the body with the other runs, and you are upping the distance meaning you're going longer than you are used to and you don't want to over do it in training. On the day of the race you'll be well rested and can go long and hard without the risk of injury that you might face when doing this consistently when tired from the previous training runs.
Yes. Intervals and longer slower [than interval pace] but still fast[er than race pace] paced runs will help with your speed. I find one interval session and one tempo session a week to be beneficial, and you'll find a lot of training plans with similar. Then add in your long run to focus on building the distance, run at slower than target pace and you'll be set!