Keep in mind that these typically rust from the inside out. Since you've got that kind of rust popping through the paint now, chances are very good that you've also got areas where the rust is right of the verge of popping through, but just aren't obvious yet.
I'm guessing the rust you're showing is right along the trailing edge of the fender? There's an up/down brace that runs along the back of the fender, and it typically catches mud and debris, which holds moisture...and rusts out the fender. Judging from your pics, I don't think I'd be out of line by suggesting you start keeping your eyes and ears open for a replacement fender, just in case the rust damage is worse than it initially appears...and I'm betting it is.
Also keep in mind that a lot of members here hadn't done a lot of repair before joining the forums...but are almost semi-pro now.

You'll never know what you're capable of doing until you bite the bullet and get your hands dirty.