If she's serious about working out, she's not looking to be picked up, BUT no one ever knows when or where they'll meet "the one". So, just start small, like smiling and saying hello. You can gauge your next step based on her reaction to that. If she seems receptive, maybe say something else, but keep it short and then get back to your own workout. She'll really appreciate that. (She's probably been hounded before, and you don't want to set off her hound alarms.)
If that goes well, talk to her a little the next time, but always be brief, and preferably before or after her workout rather than during. But try not to let her catch you waiting around for her. Even if you do that, don't let it be obvious! (shades of creepy) And don't follow her around in her workout (unless she asks) - this is the worst! Be patient, if after a few times all signals appear to be "go" (she likes talking to you and no significant other appears in the picture), then go for it!