I went through the same thing on the left hand fender apron on my car. I patched mine from the back side and then filled from the front so it wouldn't show. Here's the area before and after the repair:
http://home.earthlink.net/~pirateship/DSC20301.JPG
http://home.earthlink.net/~pirateship/DSC21003.JPG
Taking off the fender is a bit of an issue. The fender is adjustable forward and aft, so to avoid problems with figuring out where to put it when you put it back on, you should probably drill two tiny holes through the fender lip (between the mounting bolts) and through the inner apron. Then you can use brads or finishing nails as dowels to relocate your fender exactly where it was. This will make it a lot easier to reinstall correctly.
I talked about all of this on my
More Power Webpage .
After it was all said and done with patching my inner fender, I felt like it would have been just as easy to replace it. Depending upon the rusts location, you may be looking at replacing the forward fender mounting bracket too.
If you've never done a panel replacement before, this one would be a good one to start with. Good luck with whatever you decide.
BTW...on my webpage you'll note I welded my with oxygen/accetylene. Take my word for it, MIG is a lot easier.
Phil