I workout in a park about 3 km from my house, and while i'm working out, i often note some girls looking at me. My usual reaction is to just respond to their eye contact and smile - some of them smile back, others pretend they weren't looking - but now i want to start approaching them. How should i proceed?
Should i mention that i saw them looking at me?

Should i use a direct approach or try to start a casual conversation?

And also how physical can i be? 'cause i'm usually all sweaty because of the exercise...