Mike, in my experience as a fitness trainer (and a keen hiker) consider the basic "FITT" principle to build on your cardio (as long as you have no health limitations that you need to consider).
The FITT principle is:
F - Frequency (how often you excercise)
I - Intensity (how hard you work)
T - Time (the duration of each session)
T - Type (what kind of excercise)
If you do not build up your cardio fitness level over time, you risk injury to your body, and sickness (over-stressing the heart is hard on your general health even for young healthy people).
FREQUENCY: To build your cardio fitness level, most people will want to plan 3-5 sessions per week of aerobic excercise. Aerobic excercise means "steady-state"....rather than stop-and-go.
INTENSITY: You can estimate your MAX heart rate with the calculation (220- YOUR AGE = MAX heart rate). Then your target heart rate is calculated by multiplying the MAX HR x PERCENT). It is commonly recommended that a beginner workout at 50-70% of their MAX HR. Whereas a conditioned individual might aim for 70-85% of their MAX HR. In any case, you should be able to talk during excercise, without being too flushed or laboring for breath.
TIME: Cardio training is best achieved if you start with shorter sessions of low intensity aerobic excercise and then build on your time. If training for a long hike, then the TIME should be something you build on, rather than focusing on intensity.
TYPE: Although cross-training (shaking things up by trying different types of aerobic excercise) is recommended to prevent boredom, and for the sake of injury prevention (this reduces the chance of over-training certain muscle groups), you do want to focus training sessions around the type of goal you have. In other words, lots of skipping rope will get you in good shape for a boxing match, but not a long hike.
So the best thing you can do is to start getting outdoors and plan regular hikes! Schedule these on your calendar, as you would with anything else that is important to you, and stick to it.
Good luck! I'm sure that others will have some good practical advise for you also, but hope this helps.