The first 4 weeks of this plan are meant to help you transition you into a regular structured training program, They are primarily meant to prepare you for the harder work ahead and may not feel particularly challenging in the initial weeks.
The first workout you find your lactate threshold (LT).
Before we can start improving your fitness, we need to see how fit you already are. To do this, I'll have you perform two field-tests to determine both your critical intensity (CI, your highest sustainable exercise intensity) and your lactate threshold (LT, the intensity where you start to rely more on glycolytic energy production, rather than mostly fat oxidation).
Use one of the following protocols to determine your LT, which will be the upper heart rate (HR) limit for your aerobic capacity building workouts. For more information on Aerobic Capacity and Lactate, threshold you may reference the book “Training for the New Alpinism” by Steve House and Scott Johnston (note: the LT is called the aerobic threshold and the CI is called the anaerobic threshold in their book, but both refer to the same physiological responses). You may also contact Dr. Ferguson if you would like to gain a more scholarly perspective on the subject.

1) LT Test
We would like you to use the first test listed below only if you have engaged in a regular low-intensity aerobic training program for over one year. By regular we mean a structured program of training at least four days a week for extended periods of at least 30 minutes of continuous aerobic work in each training bout.
LT test: Walk, jog, or run on flat ground (or on a treadmill) easy for 10 minutes to warm up enough that you're starting to break a sweat. Then close your mouth, continue to breathe normally, and continue to increase the running/walking pace to the point where you can no longer breathe only through your nose. Back off the speed and hold this pace for the rest of the 20 minutes. This will also correspond to the upper limit at which you can carry on a conversation without needing to catch your breath. In your mind, make a note of what this intensity feels like and also note what your typical HR is at this intensity (check your HR monitor for this). This is your lactate threshold (LT) pace and HR.

2) Maximal aerobic function (MAF) HR estimation to determine LT.
If you have not engaged in the type of aerobic training referred to above or have been involved in a training program utilizing regular bouts of high-intensity training such as CrossFit, P90X, Tabata or other gym-based interval protocol type workouts, then use the following formula to estimate the top of your aerobic zone:
180-age=MAF HR.
You should also perform the first test as a comparison but use whichever is the LOWER of these two HR values as the upper limit for ALL your aerobic training unless otherwise instructed in the workout info.

Note: All aerobic training for long duration activities (i.e., mountain climbing, big game hunting, alpine climbing) should ideally be done on foot. Cycling and swimming, while great exercise, are not weight-bearing exercises while running and hiking is much more specific to your sport, which requires you to perform on your feet for extended periods.