That's right, the crowds make all the difference...I did the Gt North Run in 2006 and the London Marathon in 2007 and I was deeply puzzled by never covering the 'race' distance in training.
I can tell you though, the plans work, just follow them as closely as you can and don't worry too much if you miss a day or two here and there. It isn't like saving up to buy something, you don't need the exact amount, you need to be close.
The furthest I ran in marathon training was 20 miles, but in one week near the end I did over 60 miles in total. I had to stop for a rest when I realised that.