/ˈfɑrtlɛk/ Show Spelled Pronunciation [fahrt-lek]
a training technique, used esp. among runners, consisting of bursts of intense effort loosely alternating with less strenuous activity
Use of the fartlek technique will allow you to build your distance and aerobic capability faster than simply walking or running alone.
Especially when just starting to build fitness, it is difficult to have enough stamina to complete the minimum 30 minutes of aerobic activity by running. Fast walking at a one-second stride (think "one-stride-thousand, two-stride-thousand" as your cadence), interspersed with jogging or, if possible, actual running for 100 or so yards, followed by a resumption of the one-second pace will allow you to cover distance much faster than walking while giving you a much more extended workout than simple jogging.
Five miles in seventy-five minutes should be your initial goal.
Once achieved, add a 25-pound pack plus the "walking stick" of your choice.
Keep at it -- three times/week for the first two weeks, followed by 4X/week thereafter.
What? You don't think you'll need cardio-respiratory endurance for what is coming in the months ahead?