Basically, if you feel like your 'easy day' paces are too easy, the solution during the endurance-building phase isn't to just run faster those easy days, but to add overall mileage to your weekly training volume until your legs are somewhat beaten down and the pace drops on its own.

Of course, you should be considering this plan as a whole - if the added volume ends up compromising your hard/big days due to fatigue, you have to reconsider, but generally, you won't have the problem of easy days being too easy/boring during volume rampup training phases.