Walking for your fitness is free too although, there is the option of using a treadmill.
Personally I take advantage of using both.
Over the years I've had the opportunity to purchase bargain treadmills that do the job just fine.
I do love being out doors though.
There is something about the fresh air and being in the sunlight that just clears your mind! Which ever method you have access to it's not hard to increase your endurance.
All you need to do is increase your distance and the time you spend walking.
And if you feel that you need more of a challenge you can step up your fitness to running.
I'm one that doesn't have a lot of time to dedicate to my workouts.
Although I do them on a regular basis I also keep them short about thirty minutes.
So one thing that I have done to make my treadmill workouts more interesting is to walk on an incline.
I've also added light weights (one lbs).
Walking has lead me to wanting more out of my fitness routine.
Mainly toning my muscles.
I have always felt that you need the combination to have a complete fitness routine.
Working out with weights has many benefits as well.
So again over the years I have created my own collection of exercise routines.
I continue to purchase new ones from the trainers and programs that I feel are effective.
There have been some that I haven't thought too highly of but the ones I have kept give me the results I want.
By building my own fitness collection I don't get bored and it creates a constant challenge for my body.