You can ride for exercise on a mtn bike as well as you can on a road bicycle. You can even ride centuries and 200K brevets on a mtn bike (I've done it). If you can't distinguish a road bicycle from a mtn bike by sight, it doesn't really matter for you at this point in time. The important thing is that it fits you, and isn't going to break down every few km.
Does it fit you? If so, then fix it up and start riding.
And if you think you want a Road bicycle, go visit some bicycles shops and online shops, and find out what a Road bicycle is so you can identify one and determine if that is indeed what you really want.