So I've done the marathon. Three hours 28 minutes and 52 seconds. Just inside my 3:30 target and just outside the top 10 per cent of runners overall. I started writing this on Monday morning and was still as stiff as a board, sunburned and sore all over.
The first fifteen miles were fine with my second 10 km my fastest. Between 15 and 20 it started getting rough and then...
Everyone tells you how tough it gets at the end but nothing can really prepare you.
When I got to 22 miles I was still three minutes ahead of my target pace but every few seconds I was being overwhelmed by the need to stop running. Between 22 and 23 I did walk for a minute and then between 23 and 24 for another 30 seconds until someone in the crowd caught my eye and urged me on.
I think the final mile must have taken getting on for 10 minutes and even at 400 metres I would have happily paid £10,000 to have been transported to the finish.
As the pain subsides I can remember the high points. Tower Bridge at the halfway point and running between the skyscrapers of Canary Wharf as you get past 20 miles are particularly memorable.
I found catching up with novelty runners a good motivation. I was determined not to end behind Scooby Doo (nine miles), Spider Man (15 miles), Elvis (20 miles) or Superman (21 miles and again at 24).
One the things that kept me going was all the kind people who have sponsored Oxfam through me so thanks to them for helping me over those last awful miles. With the additional cash and cheques I was given over the last couple of days I made the £2200 target.
It was the fundraising that won me a compliment from Conservative Leader David Cameron when he spoke at the RSA on Monday morning. As a lifetime Labour Party member this was a bit strange, but being chief executive of a fiercely independent organisation like the RSA it can only be good that a leading politician chooses to join our debate about 'pro-social behaviour'. The Cameron speech was a useful contribution to the debate.
Sometimes the discussion about how we encourage people to give more back can seem rather woolly and it certainly tends to get treated that way by the political media.
I think it can provide the basis for a more relevant type of politics, with right, left and centre variants of the analysis depending on your views of social justice and the state.
But anyone who has a tendency to under-estimate the contribution to social change that can be made through voluntary collective action should have been in Greenwich Park or anywhere between there and the Mall on Sunday.
The charitable efforts of the runners of all different ages and backgrounds - many of whom ended up running and walking in all kinds of costumes for five or six hours in a baking hot day - and the incredible support of the hundreds of thousands thronging the route reminds us what sacrifices and celebrations we are capable of when we turn our minds to it.