First of all 2 1/2 marathons in 7 weeks! The rule of thumb is for every mile you race you should take one day off. After you run a marathon you should also start slow and work your way back up to running another marathon.
Also, 15 mi to 20 that's a big jump. Use the 10% or even 15% rule. I suggest adding at max 2.5 miles not 5. That is too much at one time.
