Thanks DD!
Magda, if you specifically want to run the London Marathon it's getting a place that is the hardest bit to start with. I have got a place through my charity but had to pledge to raise a minimum of £2000 which I've done (and then some!) otherwise you have to enter the ballot which opens in April (think it's the 26th) usually fills up within 24 hours. If you have a charity you would like to raise money for, have a look on their website and see if they have places for the marathon.
You'll have to start training in October at the very latest and can find beginner programmes online. It's not easy but the sense of achievement is phenomenal. Hope you can get a place and be in the position I'm in this time next year! xx