I have the same background as WorksforMe, but am just a few years older.
I like the non-impact factor of the elliptical, and use it three or four times a week. Strangely, I have some knee and ankle pain from the once a week or so long hikes I take in the Blue Ridge. The pain doesn't come from running, but from the hikes, which have rocky footing. But the 15 mile hikes are fantastic for me mentally as any thought of work (I have a high pressure professional job), so I put up with the pain and turn to the elliptical which eases it. I still run 4 times a week, paying far more attention to form and relaxation than I used to.
When I get on the machines, they are usually at a fairly low resistance level as the health clubbers who use them set them at low resistance. I warm up at this level for a minute or two, and really crank the resistance high. Yes, it still does not approximate running, but if I pay attention to posture and leg drive, I feel I am getting a good workout. My barometer is hiking - even the longest ascents in the area generally are not difficult, although with a backpack some indeed require firm exertion. I agree with the 65% rule posited above - 40 minutes of elliptical (and I am too impatient to go slow even at high resistance) equates to about 25 minutes of running.
I also feels none of this applies to the younger people here who are trying to run seriously ran than stay in shape. You need to run - and a lot, and strike that balance to build up mileage over time and avoid injury. The only cross training exercise I did in my fast college days was a rowing machine. This wasn't for fitness, either. Medium rowing efforts loosened up my back and hamstrings and I just feel looser and more relaxed the next few days. It may have been psychological as much as physical, but it was my only cross training habit.