Next time dont wait 2 months to talk to her, it shows that you have no gut.
Simply just go to her, tell her to take off her ipod and say:
"You know, I've been coming to this gym for almost 2 weeks, and I gotta say you're the 3rd cuttest girl i've seen at this gym. You would be the second if you weren't be shy and actually come talk to me

"
And dont ask her out if you're not sure if you build enough attraction, testing the water first before you fish. Try to put up some kino with her, if she goes along, dont ask for number.
Act like you got a phone call, go out of the room to talk, come back while you're still on your phone, grab her arm, drag her out and hang up the phone. Then tell her she gotta help you, you forgot it's your sister bday, and your dad/mom just called planning her surprise bday in 2 hr, ask her to help you pick out the present.
(DONT LET GO OF HER while explaining this) -- Trust me this is fieldtested, most of the time she will help you, even she has something that day, an hour at the mall wouldn't hurt her that much.
After she help you, say thank then ditch her.
Next time you meet her, "hey i was in a hurry yesterday, didnt properly thank you, how about go to blah blah, my treat"
There is your Day 2.
PS: DO THIS ROUTINE BEFORE SHE SWEATS HER ASS OFF, obviously she wouldnt want to go to the mall looking all sweaty