Weight - 89 kgs Height- 178 cms Goal - fat loss and prep for difficult treks and 21 kms marathon (these are my long term goals)

Keeping this in mind, how should I best use treadmill time?

Day 1 - Cardio, I jog/run/walk for 30 mins (able to cover 4.70-4.80 kms) for now, then I do cycling and elliptical for 10 mins each, then abs exercise

Day 2 - Upper body workout, I've been told to do 15 mins cardio at the end, I do it on incline but manage about 10 mins. I walk at 5km/h, increase incline by 5% every 2 mins up to 20%.

Day 3 - Lower body workout and do the same treadmill activity as upper body workout.

I keep seeing a lot about HIIT workouts but I'm not sure if these are aligned with my marathon/trekking goals. On my cardio Day, I run at 10 km/h for the first 15-16 mins, then walk at 6.5 at 5-6 mins, then back to 10km/h for the rest with some sprinting (14km/h ) added in the last 90 seconds. Is this a good workout or could I be doing this better?