If it's a men only run then that's fine - they are entitled to organise a male only run, the same as women are entitled to organise a female only run. But if they are allowing girls under 12 to run then they are surely falling foul of the equality act, as they are discriminating against women over 12 based on age.
There is no shared protected characteristic between all men and girls under 12 that is not also shared by women over 12 and therefore the exclusion of some females but not others is unlawful.
As a democratic society with a robust equality act, we should not be allowing men to discriminate against adult women.
Either all women or no women, or its an under 12s only event and no one over 12 may enter, but girls under 12 and men of any age is unacceptable within the law and allowing it is allowing misogyny.