The Mt. St. Helens hike is a month away, so today I did a "simulation" treadmill workout. As you know if you read this page with any regularity, I'm caught between trying to do hikes to prepare for the big trip and the lack of sufficient natural slope. A treadmill can't really give you the experience of stepping over tree roots and up boulders, but at least it has an incline button.
Of course, it's not exactly a perfect simulation. While an 11% incline at 3.0 miles per hour for 1 hour and 40 minutes may be an approximation of the climb up, the gym's treadmills don't have negative incline setting for the trip down. (I've seen treadmills that do, but never with a decline of more than 5%.) 3.0 mph is also way faster than we will or could hike. Furthermore, I'm not carrying the weight of a backpack. And at the end I did not stop and take pictures, there was no tang of pine in the air, I never stopped for a snack, etc., etc.
Nevertheless, as an endurance test it's a reasonable benchmark, I think. And the one bit of realism it has over actually going out and doing a hike is that the temperature and humidity in the gym is a lot closer to Mt. St. Helens than Tennessee in the summer is.