Catherine-
If you had been running consistent 40+ mile weeks, you might be able to get by on 1 or 2 15 mile runs for a marathon. But I looked at your training and you have a high week of 25 miles (twice), a low week of 5 miles, and a total mileage of 165 since the start of June (which works out to less than 12 miles/week). I would say that is inadequate preparation for a marathon. And I think I am being generous when I "just" say inadequate. In other words, I would very, very strongly encourage you to NOT attempt a marathon.
I would suggest you slowly build up your mileage over a long period of time so a marathon is more survivable- maybe aim for one next spring(?). Then, you actually might enjoy the experience rather than getting injured or hating it so much that you quit running. And do some half marathons along the way to help, plus a number of 20 mile runs. There is no rush to do your first marathon- build up properly so you enjoy it.
I think lots of people finish a half marathon and think, "That wasn't too hard. Maybe I can try a marathon." What they don't realize is a marathon is probably 3 times as difficult as a half marathon, esp. if you have not done adequate training/long runs. So this would be similar to your 15 mile run- imagine doing 3 of those in a row (or at least 2.5). From an exertion standpoint, that is what you would be attempting.
Bottom line- I definitely would not try it. That being said, if you decide to do TOU marathon, I would look into a Jeff Galloway-type of run-walk program. A run-walk of maybe 5 min:5min may be your best chance to finish. And I would guess your time would almost definitely be 6+ hours, given your lack of long runs and base miles, not 4.5-5 hours.
For the record (and for comparison sake), my wife tried her first (and only) marathon in 2005 with inadequate training- ran 2-4 days per week, long run of 14 miles (I think), weekly mileage of about 8-20 miles, with a half marathon a month before in 2:21. During the marathon, her body hurt so bad that she had to walk the last 17 miles of the race and she finished in over 6 hours. And she couldn't bend her knees for 4 days or walk normally for almost a week afterwards.
My two cents, which is two cents more than it's worth