Walking is generally a pretty average way to do cardio as the intensity is rarely there enough to challenge the heart and lungs enough to strengthen either of these organs or to burn many calories.
If you can't run, cycle or swim however, then walking may be the only option available to you. What you may need to do is to boost the intensity of your walking in order to achieve weight loss results.
You could of course just increase the duration or the length of your walk, but you might find it taking up a sizable part of your time and by adding a few hills into your walking routine you can boost the intensity and the effectiveness.
Sample Walking Program:
Find the biggest and steepest hill in your neighbourhood
Walk at a steady pace until you get to the hill
Get to the base of the hill where and start your stop watch
Get up that hill as fast as possible!
Check your time.
Then walk back down
Do this 5-6 times aiming to improve this over time
Slowly walk back home
Hot Tip: Over time build the amount of times you go up the hill and try to reduce the time it takes you to get up it.
Join a Squad or Group to Stay Motivated:
It's much easier to do your cardio in a group or a squad format. In walking, running, swimming, cycling squads you will find like-minded people who are just like you wanting to get fit and do it with a bit of guidance from a coach. You will find you will easily do twice as much with what seems like half the effort when you're in a squad which is a huge benefit.
Even though you may never have any intensions of doing a triathlon, you will find that many triathlon squads cater for all levels of fitness from the very beginner recreational athlete just starting out to the more serious athlete and for those of any age group.
You will also find that if you like swimming you can just join them for their swim sessions, if you ride... join their cycling sessions and if you run...join their running sessions. This way you get the benefit of a group environment even though you may not do all 3 disciplines or have any intention of doing a whole triathlon.