I’ve done the intro to ultra-running plan, and it is specifically for a 50k. Since your event is longer, I’d use the Big Vert plan as it scales to your goal event distance and elevation. I”m sure others could chime in with advice specific to that plan.
Since the Big Vert plan is 20 weeks, you would be starting it towards the end of March, so you wouldn’t have much training before the half marathon and would need to be smart about modifying your training load in the weeks before/after the half marathon.
Only you know your actual fitness, but I think a 4-day Emmons climb in May is already doable if you are hiking and running that much. Again, you’d need to be smart about modifying your training load pre/post-Rainier.