I've recently bought Microsoft FS 2002 Pro.  What I would like to ask, is how to judge your horizontal distance especially with regards to the distance to the runway / airport

In all the tutorials, sites i've read so far, it just states start your descent about ... nm from the airport.  How do you do this just by using visual references, and not using nav aid beacons, GPS etc?

I would especially like some input from "real-world" pilots.  

Thanks

the first thing you need to state is, what plane? and also what altitude are you flying at? its kind of difficult to tell how far you are from an airport using just your vision, as there is no real sense of depth in flight sim, its difficult enough in the real thing, it took me untill my third lesson just to learn where the 20 mile mark for the airport airspace was at bristol and thats using landmarks basically

If you have VFR charts of the airport, you could navigate by terrain marks, but they aren't that accurate

As far as purely visual cues, unfortunately, Y2cragie is correct. Map mode, in views, can be a good reference, or you can use the distance rings on the GPS map view(with the range zoomed in). I'm assuming your starting with the 172/182, or something of similar performance. Also try using an airport that has a VASI(Visual Approach Slope Indicator). These are the red and white lights on the side of the runway. Not all airports have a visual lighting system like this, but using these for practice will help you establish a proper sight picture. If you  see two red lights, that is bad, you're too low. Obviously hitting the ground is bad, but seriously, a flat(low) approach limits you're forward visibility and may neccesitate the need for a lot of power. Two white lights means you're too high. This isn't quite as bad, as long as you have a long runway.  One red and one white light indicates the proper glidepath. To land at the edge of the runway "on the numbers", you should actually aim for a point just short of the runway.
                                     Good Luck!

As a follow up to yesterday's advice. First of all, after re-reading what I typed, just so that there's no confusion, a VASI consists of two SETS of lights, not Two lights. After I tried my own advice at Meigs field(which has a VASI in one direction only), I found it hard to discern.
Runway 28L at KSFO is a much better location. This runway has a PAPI (Precision Approach Path Indicator). Don't worry, it's the same basic concept. This runway also has two excellent landmarks, a bridge at about the 5-mile mark, and a peninsula(very easy to see)around 2-miles.The PAPI consists of four lights side by side. From about 5-miles the lights will appear as a white line perpendicular to the runway. As you get closer you will be able to make out the four indivdual lights. For proper glide path, you should have two red and two white lights. All red- you're too low. All white-too high.

You pretty much go by landmarks.   With the airport I fly from we have a highway a mile out, a mall 5 miles out, a reservior 10 miles out, etc.  Without using nav aids, you judge your distance using this.  How would you do it in IFR conditions?  Use nav freqs or the GPS.