To maximize this bike and get it's full potentiall you must use it

... Just kidding, sorry

....
I just Googled a pic and it looks to be a decent bike and all that it may need is a tune up and that's it, it may be old but everything Retro is coming back... Don't be afraid of using the higher intensity, you will NOT get manly legs, that's a myth... It'll give you a more difficult cardio and that's a good thing, it'll burn more calories. In order to get manly you need a high level of testostorone and most women don't have too many of those hormones.
So, my suggestion to you is to clean it up, tune it up and exercise away... Good luck keep us posted on your progress