The Rat - What mileage have you been doing per week up to now over the last 3 months? The Furman schedule is best entered into with at least 20 miles per week of good quality running - ie tempos, intervals and long runs (your max of 13 is ok so long as it wasn't a one off and outside the last month).
However, don't fret - your time of 1:30ish is good particularly considering your training to achieve it must have been sub-optimal (normally your longest run would be the race distance plus 20% EXCEPT for the marathon) - so clearly you have natural talent and endurance which can only be a good thing for the marathon.
What worries me most is your enthusiasm!!!! Excessive training, considerably higher then normal workload with greater speed leads unfortunately to injury (unless you are very well biomechanically structured).
I suggest you for now at least stick to the Furman schedule and ditch your additional weekly session. Give yourself 4 weeks and if you don't develop any persistent or growing pain then add in the additional session and monitor carefully. BUT you don't need the additional sessions if the 3 key sessions are performed as directed and the 2 cross training is tiring but not killing with 1 recovery (slow and easy 4-5 miles)
I suggest you set an A, B and C objective (A dream time, B strong target, C good time) For you I suggest a 3:10, 3:15 & 3:30. Which are all very respectilble times and you might just be surprised if your training sticks to schedule and you don't get injured.
I pass on this advice based on 5 years running for which I have continuously and steadily improved based on an interval, tempo and long run principle just like the Furman schedules.
My targets are sub 2:45, sub 3:00 and sub 3:15 - which are targets that are achieveable based on form, worthy and good for age (so I can try again next year without the ballot to reach my potential!). This is my 2nd marathon and I have alot prove - the last was in 1996, when I popped at 21 miles and finished in 3:35 - in those days I was more a footballer than runner and my compromised training was my undoing.
Look forward to your response.....