Mt Rainier is a great mountain to climb and gives a good opportunity to travel across glaciers and navigate crevasses.
When training for a peak like Rainier it’s good to consider the requirements of the climb. The DC route has 9,000 feet of elevation gain. If done in a 2 day climb with a Muir camp your day 1 will be 4,700 feet of gain with an overnight pack (probably 35-50lbs) and a summit day (hardly another day since you wake up around midnight or before) of about 4,300 feet of gain with a light pack usually less than 20 pounds. This day will also include descending back to base camp and then all the way down the mountain. It can easily be a 15 hour or longer day.
Another popular option is a 3 day trip where day 1 is the same but on day 2 you move your camp about 1,000 feet higher to Ingraham Flats just below the Cleaver. This short day can give you some nice rest before the summit day and give your body more acclimatization time.
Probably the most important training component of your program will be your pack carrying. You definitely have good training areas where you live. You’ll want to progressively ramp up your pack weight (ultimately to at least your planned Rainier weight and possibly 5 pounds more), duration (time or mileage), and speed of weekly training hikes.
In addition to regular pack hikes for several hours and an in-town 1 hour or so pack session it’s a good idea to have a few
back to back pack carrying outings. In order to properly prepare the body and mind for consecutive days of hard training, you need to simulate that training by carrying a pack two or more days in a row. That way you will learn how to work on less rest, even though you may feel a little sore or tired.
Most people that gravitate toward mountaineering already have been involved in quite a bit of aerobic activity however they are often much less experienced in strength training. Typically strength training will be the second priority in your training. Two to three days of full body training should allow for good strength gains.
You can read much more on
Mountaineering Program development on my website.
Good luck.