Your stamina is already very good. You should be attacking for speed and power gains. The point isnt whether you can handle the volume. The point is to get faster, and drop your cruising speed.
Even if you want to go the stamina route over the summer, you'd be better off piling up more medium hard training. Try adding in more 530-545 miles and dropping the volume down to a level where you can do some tougher work.