Howdy,
How many times have you heard this or similar question? If you ask me soooo many… So it’s my turn to try to simplify my thoughts on this not easy question.
Many people think that for better and faster fat loss results you should run as fast as possible as long as you can. In one hand perhaps it’s close to the truth because of post exercising burning process (calorie consumption). This is one interesting phenomenon exists only at high intensity exercising. After HI workout your organism continues to consume oxygen who was lacking during workout and need energy for that and several of other physiological processes (if intensity and volume both was high, it may take more than 24 hours). For example, after HI marathon race higher metabolism stay in game really long time and full recovery lasts about 7 (it’s a so “magic” number – you will see soon 🙂 ) days. But it’s really not necessary to be masochist, your workout should be effectively comfortable!
At high intensities energy expenditure is reserved for carbohydrates dominantly. But during recovery fats are included more – which the main target is when it comes to fat loss.

A complete masochist 🙂
When it comes to “running or(vs) walking” topic, 7 is a “magic” number you should try to remember. 7km/h is approximately transfer speed from walking to running (about 2 meters per second). Don’t take this “rule” for granted because it depends on personal tallness (guy with longer legs-steps will feel easier to walk on higher speeds). But for majority of people 7 is a “magic” number. You can check this on treadmill. Start to walk, move up your speed progressively several times and notice on which speed moment you feel it would be easier if you start to run instead of walking. That’s your “magic” number 🙂 .

Find your “magic” number on the Treadmill
What I wanted to say is that any walking below 7km/h is MORE ECONOMIC than above same speed. Otherwise, any running below 7km/h is LESS ECONOMIC than above same speed. Ahmmm… I guess you are little confused? 🙂
Don’t worry – I will simplify this with getting more precise. If you run below 7km/h it will be a little harder for you than if you walk below same speed. And if you run above 7km/h it will be a little easier for you than if you walk above same speed. That’s mean that slow running and fast walking both are the best option if you want to burn calories on most efficient way! Remember that if you want to get slender faster LESS ECONOMIC is one of the main targets!
What to choose in a practice is second important thing. If you ask me, my first option is FAST walking because of safety and better muscles recruitment (if you feel ready enough – walk as fast as possible without technique worsening). When I say safety I mean on less contact forces especially for obese people. During walking flying phase does not exist (less bad-for-joints contact forces). They (obese people) should swim at the beginning because this is the most safety no gravity position for them, plus when you swim you use more than 2/3 of your body muscles which is important for burning more calories. Also, first contact with walking is on heels – that’s mean better glutes and hams recruitment with minimal tendons elastic energy utilizing comparing with any running type (that’s mean more calories burning). When you walk fast your front heel contact is a little further ahead (more in front of vertical hips projection compared with slow running) which contribute with better (longer) posterior chain muscles involvement. In other words, fast walking extends step(s) (legs are longer on the ground which means longer posterior chain activation). Braking forces (with front leg’s first ground contact) are also higher but it’s even better because the posterior chain has to start system quickly and to make a fast propulsion. ALL IN ALL, MORE ENERGY IS CONSUMING WHICH IS GOOD FOR HIGHER CALORIES EXPENDITURE – which is very much useful if it lasts enough long (or if it lasts less for the same effect like a longer duration)…

Walk fast! Bent elbows are better option because with extended elbows you will maybe have longer lever (LL = harder for muscles = spending more calories) but pain in the front of the shoulder may occur with a greater range of motion. Anyway, maintaining elbows bent = spending more calories again 😉
Also, “slow = boring” for most people – that’s mean they will rather choose fast walking than slow running (with no straight line for sure 🙂 ). It doesn’t mean you need to avoid any running speed if you are young recreational and if it’s comfortable for you and fulfill your sports spirit – if so just keep running!
In short, slow walking doesn’t have any positive influence on heart health because it can’t raise up heart beats significantly (it works only if it lasts for hours but only on fat loss – women during shopping can burn some serious amount of calories 😉 from fat stocks). Fast walking has pretty good influence on increasing heart beats – heart health and burning fat cells too. Slow running is not so bad, and fast running isn’t bad at all but only if your body is properly prepared. “Don’t run to get in shape; get in shape to run.” – Mike Boyle. Fast walking at a slight inclination is maybe the best option (but be sure not to bend over too much – I know it’s easier but important back chain muscles work less on that way). Running under an inclination could injury your Achille’s tendon because it could stretch it more under more load so I don’t recommend it for poorly prepared people.
Fast walking in the nature (not common treadmill) is always a way better (not only because of the nature influence). In one of the following posts I will explain why.
So, find comfortable speed and enjoy your wonderful fat loss process. Find your “magic” number and walk above it for maximizing effect!
Thanks for reading and all the best,
Luka