Increasing Mileage

I have to increase my weekly miles from 36 to 40 for the next step up in stress. On the surface it's about a 12% increase. But it's not that simple. You see, I was doing hill repeats 2 mi from my house. My base run was 3 and 3.5 mi. To get to 40 mi per week I have to have a base run of 4.5 mi, a 150% increase in my daily run. I base my interval, threshold and long runs on the base run. The number of .5 mi intervals is (BR-2)/.5 because I take 1.5 mi to warm up and a .5 mi cool down. The threshold run is BR-2 at 135-145 b/min heart rate. Long is 3 x BR. Next week I run a base 4.0 mi, 4x.5 mi intervals, 2 mi threshold, and 12 mi long for a total of 36 mi.