Debbie,
There are various ways people use to calculate Max HR and the age predicted formula is one of them. It is probably the least relaible. If you are using a Heart Rate Monitor the instructions probably have a method described that may require you to determine your resting heart rate and adjust the Max HR based on this.
Here is the Karvonen method which is adjusted for resting HR:
http://completerunning.com/archives/2006/10/27/the-karvonen-method-heart-rate-zones/
These methods too are not completely reliable especially for more fit individuals. You can get Max HR tested professionally based on an exercise test as an option. However, Karvonen or the mehod described in the HRM manual may be a more convenient option.
When you are doing sprints, it is suggested to use speed and leave the incline at 0%, and for hills it suggested to go slower and use incline. A trek is a combo of hill and sprint where you use moderate incline and speed in combination.
One example could be Level 3:
Sprint - Run at 5.8 at 0%
Hill - Walk at 3.8 at 10%
Trek - Run at 5.3 at 1.5%
Another alternative:
Sprint - Run at 5.8 at 0%
Hill - Run at 4.9 at 3.5%
Trek - Run at 5.3 at 1.5%
The numbers are just indicative, they should depend on what feels right for you.
During Level 3, you should feel like you can just-about-manage a few quick words while walking/running, not pushing so hard that you would feel annoyed/unable to respond at all if someone came up and asked you a question, nor working so easy that you will be glad to chat. You may want to try to find a setting for which within 30 to 40 seconds you move beyond comfortable, but not so hard that you feel you will be forced to cut back within less than a couple of minutes of starting that level. If when you enter Zone 3 HR, that feels too easy or too tough compared to this description, the Max HR formula may not be working for you.
~* Vrinda *~