This is where rights conflict.
Women and girls have a right to not be discriminated against, including Muslim women and girls. The fact that girls over 12 are completely banned with no alternative is discriminatory.
Some Muslim men find it offensive to their beliefs and dignity to share activities with women and girls. Some muslim women will also feel very uncomfortable running in the presence of men.
Discrimination is allowed to happen if it is a means of achieving a legitimate aim, ie increasing sports participation amongst a sex historically under-represented (ie womens only run clubs, gyms, swimming schools). Given that Muslim men are free to run whenever, and can also donate money whenever, I don't see how banning women and girls over 12 meets this rule, when they could sex segregate.
The real question is to Tower Hamlets council, how they are allowing their park to be used for this event, but given as its Tower Hamlets, I wouldn't expect any less from them.
The EHRC is on the case and I will be interested in their opinion.