Well no, I wouldn't say you were silly at all to have a time in mind - it's difficult not to - but just be flexible with it, so you don't feel disappointed if you don't manage it. Also, having a time gives you something to aim for if you do shorter races between now and then.
I had the magical 4 hours in mind when I did Paris - but I was way off. I was disappointed, which was stupid really because running a marathon is a huge achievement whatever time you do!
In my experience (and I have only done 2 marathons - so I'm not an expert by any means!) more miles doesn't necessarily mean a faster time.
For Paris I ran up to 45 miles a week, I think that was over 4 or 5 runs. I did one 20 mile run and a couple of 18s. Most of the runs were at a slow pace. I ran 4:19.
For London in 2007 I ran 3 times a week, my maximum mileage was 35 miles in any week - and that would have been a week with a 20 mile run at the end of it. I did one interval session, a tempo run and a long run. I did 5 x 20 mile runs and a couple of 18s. I ran 3:39.
Now, it's difficult to compare properly because I was fitter when I started the 16 week training programme in 2007, but for a couple of years I thought I had reached my peak in terms of race times - until I did the intervals & tempo runs (never done that sort of thing before). In the run up to the marathon I took 10 minutes off my half marathon time (to 1:35) and 3 minutes off my 10k time!
Thing is, different training works for different people. If I go much above 35 miles a week (and I have tried it a couple of times) I start creaking and get injured. Other people thrive on it.
I have read lots of stuff to suggest that doing lots of miles at a slow pace will mean you can run lots of miles but you will be slow. I agree with that and think including some faster paced training definately makes a difference.
Oh, and when I did my 3:39 - after the 20 mile point I had to walk at every mile marker between there and the end! I would get a bottle of water, have a walk and a drink, then run again - I was absolutely knackered. Again, there are some arguments that taking regular walking breaks (ideally before you are forced to walk (which I was)) can result in a faster time than if you try and run all the way around!
Mmmmm, a bit of a ramble - but I think what I am trying to say is have a time to aim for, use it to motivate you through the training, but if it doesn't pan out then go easy on yourself and try again next time (if there is one, obviously ).