Is Rainier possible?

Probably not but definitely maybe!!!!! 😉 My primary concern if I were in your shoes (boots) as a relative beginner wouldn’t actually be fitness but rather the other member(s) of my climbing team and what the logistical plan is for the climb (route choice, route condition, camp placement, rope teams etc) . That will be far more likely to be what makes or breaks the climb and makes it safe or hazardous. Setting that relatively huge point aside then the question is whether you can do it physically which I think is more what you’re asking here. I don’t actually think living in Denver is a huge advantage here for the altitude but it is for the proximity of the terrain to train on. There have been plenty of folks who have successfully climbed Rainier who live in Florida and also a huge number of folks who live in Mountain environments at some altitude who’ve gotten their butt kicked. The best thing you can do between now and then I start getting out and some steeper terrain locally (Bear/Green in Boulder, Incline in CO Springs, Quandry, etc) and getting some very specific training in to see how your body responds. Once you get about 12 weeks out or so do you want to start really training in a way that’s as much like the climb as possible and that will certainly include a fair amount of weighted pack work in the hills. Once you start doing these specific workouts I think you’ll get a much better idea of whether climbing Rainier this season makes some sense or whether it might be best put off till summer 2023 and get another year of training in. Hope that helps and I am sure some others will weigh in here!

Great goal! But hard to answer the question directly. Mark’s got the key points covered, but I’ll add a little – especially if others are considering similar goals:

Rainier’s glacier hazard can’t be replicated in Colorado. You’ll need to make sure that you and your team are prepared for that, along with the other possible hazards. While there are lots of success stories of newer climbers doing these routes (I’ve seen this on the DC and Emmons), many of those climbers often don’t realize how poorly they managed the hazards on the mountain. This “wicked” learning environment usually only provides feedback with poor or even devastating outcomes, so those with less experience simply don’t have the time in the field to actually know what they are doing right or wrong (unconscious incompetence). Hopefully you’re lining up with an experienced group and/or have some educational experiences planned.
If you are traveling with experienced climbers – climbers who’ve been through a formal mountain apprenticeship and/or education, and current with crevasse rescue and glaciated mountain travel – then you might have that technical side covered with your team. To be a contributing team member, it’s a wise idea to do a glaciated mountaineering course and try to do a few easier climbs prior to your big goal.
Alternatively, doing Rainier with a guide, a Baker skills seminar or even an Emmons skills seminar can speed up that learning process and give you practical experience. Success ratios are pretty high for guided groups, but vary quite a bit correlating with experience with non-guided groups.

Being in Denver should be awesome for your specific training. With a little driving, you’ll have some great opportunities to train in the mountains at elevation. Follow Mark’s advice on checking your progress on the specific workouts.
There’s a lot of questionable behavior on the Colorado high peaks, and as a new resident it’s good to be thorough with your homework and avoid relying on hearsay. Many of the online forums for Colorado mountains have questionable information on hazards (avalanche, early season snowpack, rock quality, etc.) and how to manage those hazards. The mountaineering groups and clubs are great to get started, but keep in mind that they regularly consult with certified guides for definitive information and best practices.