I ran the great north run and made a pledge, am i legally bound?

Probably not.

Did the company give you any benefits in return for your pledge (eg did they pay your entrance fee for the run, contribute towards the cost of kit etc)?

If not, it seems very unlikely this is a legally enforceable contract.

No, charitable "pledges" are not legally binding contracts. There is a rare exception, such as if you pledge a good amount of money that the charity then substantially relies on, but that has not occurred here. BTW, it does not matter if you signed a form because a charitable contribution, or act of gratuity, lacks the necessary requirements to be a legally binding contract.

morally yes

legally - did you sign any form? have you raised any funds

if yes then you are bound because otherwise you are not fulfilling the pledge or stealing

if you have raised funds then hand them in otherwise you are stealing from charity

I expect you had to pay £50 to enter it, much smaller races manage to do it fot £10 or less, and that is with far fewer people, the great north run is an economy of scale, so how they charge so much is beyond me.