JustGiving is the world’s social platform for giving. Their mission is to ensure no great cause goes unfunded. They have helped people in 164 countries and raised over $3.3 billion for good causes since they were founded in 2001.

You can find loads of challenges that you can support. For example Elizabeth Francis who is raising £ 10,000 for the Multiple System Atrophy by running 30 races.
https://www.justgiving.com/Elizabeth-Francis3/

She says: „I think that you underestimate the response from people. And it is not just about how much money you actually raise. It is about raising awareness. It is about getting charities out there and getting people on board. No matter how crazy it might sound, how hard it seems, just go out and do it.“