Preparation
Download a walking app in your mobile
Buy a japa mala

Method
Set a goal on your walking app ( ex: 150 calories to burn)
Start walking with japa mala in your hands and start the mantra
Stop till you burn 150 calories

Note: I finished 4 rounds of mala (108 times) of a mantra by the time I burned 150 calories by walking

Benefits
It is relatively inexpensive if you already have a japa mala
You do not have to register at any expensive gym
You can do whenever you have time and wherever you can, as you can also do in your kitchen
If someone disturbs you, you still can continue the mantra as you know where you stopped because of your japa mala
And the ultimate benefit: you are burning calories and at the same time you are establishing an ultimate contact with the God

Some may argue that chanting a mantra has to be done in a clean pure place and with ultimate concentration and devotion and that it should be taken seriously. I have seen many Lord Krishna devotees walk with japa mala chanting “Hare Rama Hare Rama Hare Krishna Hare Krishna” with bare feet walking in the airport. It all depends on whatever you are comfortable with. Mantra becoming a part of your life is what you really want, to achieve whatever you desire.

So, why wait when you can be spiritually and physically fit easily. Good luck!!!