This revelation has given me mixed feelings. On one hand I cannot wait for the big day, but on the other I am terrified. As we drove on and on I realised what a big task I have set myself. As we turned the corner at what will be mile 20 - the furthest distance I have ever run - I noticed with a jolt that there was a slight incline that went on and on. It is no steep hill, but I could tell that after 20 miles on my feet I will not welcome this increase in gradient.
I was once told that 20 miles can seem like halfway in a marathon, not a cheering thought. That said from that moment onwards there are so many treats in store that I am hoping they will keep me motivated to power through the home stretch.
There is a glimpse of HMS Belfast through a gap in the buildings, there is the Embankment snaking next to the glinting ribbon of the River Thames, there are the space age silver capsules of the London Eye, the gothic splendour of the Houses of Parliament. I am hoping the majesty of London will keep me going until I sweep around in front of Buckingham Palace to the finish line on The Mall.
For all that I know the marathon will be tough, I couldn't have asked for a better city to run it in. It always amazes me how inspiring a bit of pretty scenery can be on a hard slog of a run. The first half marathon distance I did was enlivened immeasurably by a gorgeous view across the city from Primrose Hill, while a recent tough hill run along the coast path in Devon was lifted to another level by the immense views across golden sandy beaches and glittering blue seascapes.
This week is one of rest and carb loading. It sounds like heaven after all the weeks of training, but actually I am finding short runs and eating lots rather dispiriting. I have put on masses of weight and, while I am trying hard to be sensible about this, I am hating seeing a higher number every time I step onto the scales. In fact I can't wait to finish my marathon and get back into training proper for my next half marathon in June - clearly I am a glutton for punishment.