Chicken & egg. I once started with a similar idea - meeting sport buddies - but on a country scale. I found it very hard to gain enough traction for people to actually be able to meet each other.

Because.... what value does it bring for Joe who's living in SF, that Jane from NY wants to go for a work-out? I think "importing" events from other sites is one option to get things going (still - quite a lot of effort). Other idea - is to start locally, run a big ad campaign, get in touch with sport centers, etc, then "conquer" other cities, one by one.

Good luck! :)