Dr Mike Stroud was the first Briton to complete the race. If you consider he's a doctor and an exceptional athlete and he had a tough time of it, you might want to put it off to five years or so. If you read his book "Survival of the Fittest", he describes the event in detail... including terrifying cases of heatstroke.
I'd suggest you spend at least two years on endurance training. I very strongly recommend the SERIOUS (that's an acronym) system as described in the book "SERIOUS Training for Endurance Athletes" by Rob Sleamaker. Also consider getting textbooks on sports nutrition, and some on adventure racing to see other people's experiences. Also Stroud's book covers the medical/physio info you'll need for the race.
Best of luck. I hope to run it myself one day.
MEN WANTED FOR HAZARDOUS JOURNEY. SMALL WAGES, BITTER COLD, LONG MONTHS OF COMPLETE DARKNESS, CONSTANT DANGER, SAFE RETURN DOUBTFUL. HONOR AND RECOGNITION IN CASE OF SUCCESS.
Reverend "Norwegian Spruce" Arctic Hobo of the Bushcrafti