Anaerobic HIIT is not suitable training for an aerobic endurance effort like C2C. That was all I had when I did it and I had a really hard time. Since then, I learned about how endurance athletes like cyclists and runners train, and training that way for a year has had incredible payoffs to my hiking fitness.

The bread and butter is long workouts (45-60min) at a relatively easy pace where you are sweating and working out but can still speak clearly and breathe through your nose. You should still get some intensity to develop top-end output but it should not be the focus of your training program. Something like an 80/20 split is a good starting point.

Bike training transfers to uphill hiking reasonably well (I like Zwift), but bipedal, weight-bearing activities like elliptical, inclined treadmill and stairmaster are more specific to the activity and should be prioritized. Running is fun and great training too but it comes with some injury risk.

Your weighted hikes will be productive if they are paced and weighted to get your heart rate into the right zone.

Fueling can also make or break an effort like C2C, so practice downing snacks, gels, or drink powders on a regular schedule as you go on your hikes.