I agree completely with @trailrunner85 that you can't wing a marathon. But I don't think you need 5 good training runs at 'a decent pace' in order to be successful.
I ran my last marathon at a 9.37/mile pace (and, if I'm honest with myself, I probably could have run the last 10k a bit quicker than I did), but all my long runs (18, 20, 18, 20, 22 miles) were run at a much slower pace than that - apart from the 22 miles, they were all run at 10.30/10.40. 22 miles was only quicker than that (10 m/mile) because the middle of it was a half marathon race.
I was quite strict with myself on those long runs, often having to slow down and temper my pace, being very deliberate about going at a quite sedate speed.
I had other runs at 9.20-9.40 pace, but only 5-6 miles. And I did a dedicated speed session every 7-10 days (again no more than 5-6 miles of intervals, probably averaging c 9-9.20). So I didn't do any significant mileage at all at my eventual race pace.
That approach to training took 4 minutes off my previous best, and I felt comfortable for the first 21 miles. I hadn't run anything like that pace in training and it still felt good.
I'm a total believer in running the easy miles (in training) easy. Much slower than you are used to. I finished all my long runs feeling like I could go further/do more, and my legs were fine the next day and I could go out and do a fast run after a day off. I think if you attempt to run your long runs too fast, it just takes too much out of you and you spend too much time playing catch up.
After previous marathons, I've been very doubtful about doing it all again. But this time round, taking a more relaxed approach to the long runs (and clocking up a lot more miles than I ever have before) meant I enjoyed the training process, and I'm keen to go again!