Thanks for writing in with your questions.
I hope these answers are helpful.
Your first question. It will most beneficial for your aerobic development if you stay in the aerobic training heart rate zones as much as possible. You can do this by testing like we recommend or by checking to see if you can speak in full sentences.
Second question: Your event with 5600m in 75km is an average grade of 27% on the (up) climbs if the course starts and finishes at the same elevation. This is a similar grade as the Hardrock 100mile event.
When you increase the distance during your training in the mountains you also increase the vertical. They go hand in hand. However, if you have concerns about handling this training load, by all means do not force yourself to stay with the plan. Something I talk about a lot is: DO NOT BECOME A SLAVE TO THE PLAN. The plan is at best, a good guess at what can work sometime out in the future. It is a suggestion not an order. All training plans get adjusted based on the athlete’s feeling form day to day. I have coached many very high level athletes and my main job is not laying the plan but adjusting the plan, often on a daily basis. This even when I am contact with the athlete every day. So, now you are your own coach and you are wise to listen to your own feelings and adjust accordingly.