The hike around the base of Mount Rainier is a multi-day endeavor. Since the author mentions a guide, I’d guess this is what they mean. It requires preparations like shipping supplies in advance- jeans would be a poor choice.
Author here... I edited "around the base" to "at the base." It was indeed a day hike. I originally said "around" as in "putzing around," not "circling around."
If you show up to my 140 km multi day trip in jeans you get left at the car and not permitted to come to another one. If it's a long day hike you get left at the car but a second chance. If it's a short day hike (but still worthy of a guide) you put on the spare pants in my car.
