Someone asked Rav Yaakov Ariel, Chief Rabbi of Ramat Gan, if it is permissible to participate in the Tel Aviv Marathon that is fast approaching, considering it is a mixed-gender race. He also asked the same question about participating in Triathlons, events that include running, biking and swimming, that are mixed-gender races.
Rav Ariel's answer was simply, if people were dressed properly, it would be allowed. However, unfortunately, people are not dressed properly.
source: Yeshiva and NRG
uh oh. Good thing I am not participating in Marathon Tel Aviv...
Actually, this would apply not just to the Tel Aviv Marathon, but to all such marathons and sporting events. I know people who run a lot, but don't run marathons or other races only because of the way people (of the opposite gender) are dressed..
The truth is that this also applies to most of life - walking down the street, taking a bus, going to work, gooing to a mall, a supermarket - people are dressed immodestly everywhere. Granted, in sports clothes it is a lot of that concentrated into one place..
I find it interesting he did not employ the issue of it being a mixed-gender event, and only one of immodest dress.