Of course if you do it as part of an effort to raise money it is a bit easier. Along with the aches and pains you experience as the miles build up is the psychological warmth of knowing you are doing a good thing and helping others.
Paris marathon anyone?
If you are going to undertake the task then maybe it is better to do it somewhere with nice views as you go round. Paris is a beautiful city and they have a marathon usually a week or two after London.
With it being so difficult to gain entry to the London marathon a trip to Pris is a great back-up plan.