Welcome to the Uphill Athlete forum.
I am not sure if you have read any of the Uphill Athlete books or articles on the website. If not, this article would be a good starting point to understand the basics of the Uphill Athlete training approach.
I would recommend you to do an aerobic self-assessment. This will help you determine your AeT / AnT and set the right training intensity zones (based on heart rate).
Looks like you have been doing 2-3 sessions a week on the treadmill, which is great. As mentioned in the first article, gradualness, consistency and modulation is key to successful training. Check out this training plan for trekking that can help you build your own training plan.
Hope this is helpful.