Me and my wife have done the London Marathon twice now, if you like running you will love the marathon. The atmosphere is amazing, the support you get from the crowd is so uplifting. You have a couple of options, you can enter the ballot for a place, which is a huge lottery as normally about 100,000+ people enter. We entered the ballot about 3 years running but we got no where. We then decided to run for a charity, which is your second option.
Different charities expect you to raise minimum amounts of money ranging from a few hundered pounds upto thousands.
First time round we ran for St Johns Ambulance and we had to raise a minimum of £1250 each!!
It sounds a lot but it was surprising how generous people were. You might have to be a little bit creative in your fund raising if you struggle.
The second time we ran in 2009, we got lucky and got ballot places, it was surprising how much more we enjoyed it second time around, maybe because we knew what to expect and could take more of it in. It may have been due to the fact that we didnt have the pressure of finding £2500 in sponsor money.
Either way I highly recommend it but dont under estimate how tough it is to complete a marathon! Preparation is everything!
Good luck!