I need advice on what to do for my legs and butt!?

I have stretch marks on my legs and butt and i think they are absolutely hideous. I want to possibly tone them up or something to tighten the skin and im going to use a spray tan to help as well. im going swimming this week in about 5 days and i want to know how much leg toning excising is needed to have a little effect by then. i am NOT trying to lose weight, but gain muscle mass. if it helps i am 5'9' and 124 lbs. (please do not comment on my weight if you are going to criticize me. i don't need a lecture on this because i cant gain weight to save my life) if you have any suggestions or ideas that you know help hide them i would gladly appreciate them. thanks.