I couldn't agree more with the advice given here. I decided back in the summer of '07 to begin training for my first marathon and be ready to run it by October '08. In that timeframe I've gradually increased my mileage, worked on tempo running and increased by cross-training. My long runs are now weekly at 13-15 miles and recently completed my first timed half-marathon run. But that was with a solid 6+ months of training.
Going about the training in that manner has kept me relatively injury free and has helped me dramatically improve my speed at shorter distance runs like 10Ks.
This was all done after speaking to and listening to the advice of people who have been down that path before.