I really do think you have the wrong idea
Cancer research, macmillan, age concern etc all run events, half, full and ultra marathons . You run / walk for that charity only. You pay an entrance to cover the cost of the organisation of it, timings, photos food, water etc - Macmillan who I walk for charge between £25 and £40 per entrant depending on event, how early you sign up etc
You are then asked to raise a certain amount for charity - each charity specifies the amount. I walk for macmillan and I think last one I done was £250 - I raised £2k . Macmillan NI really rely on the £1m they make each year for that one days event
I did the half marathon first and 3 years later did the full marathon aged 60 . Both were huge undertakings , first one I was fat, unfit and just recovering from covid, the second was to see if I could go the distance after a year of improving my fitness
@tilyougetenough , I raise by putting the just giving link to my page on my social media , posters and sponsorship forms in my local pub and a collection box and poster in the local petrol station/shop . During the challange I post video updates , I think the one where I was sobbing with pain and barely able to walk got me the most donations :). I also do a car boot, sell on eBay and Facebook etc.
Go and enjoy the event. They are so well organised , you are well supported , its a great introduction to walking/running. Since Ive done those I now do park run . I think I will do another on my 65th , but that's for me so I will donate the sponsorship myself