Let's go back and review on the different forces that are acting on a person marching.
In this force diagram you can see that there is obviously gravity pulling on the marcher. But the ground is also pushing back. These are the main forces. Then you have the little, but still mighty forces. Friction and momentum. The friction is keeping the marcher in place and not letting the marcher slide or roll away. The momentum is how much force the marcher is actually putting into marching. Or in other words how hard they are putting their bodies forward. This controls how far the marcher goes and how fast. Now, the lift of the foot is just to make the movement smooth. Instead of plopping your foot down, you have that slow and smooth roll. All of these forces take part in a simple move. This move is just one step and not even a full one.
In this force diagram you can see that there is also obviously gravity pulling on the marcher. You can also see that the ground is still pushing back. And there is again friction and momentum. The friction keeping the movement steady and the momentum determining how fast and how far the movement will take the marcher. But there is a slight different force. Instead of the lift of the foot there is the push of the foot. This is giving more momentum to the movement. This can make the marcher go father or faster. But it does depend on how much push you give. Then you will receive the equal and opposite push back from the ground.
So the acceleration of marching is just the change of the speed of marching or the change in direction of marching. You can actually calculate the acceleration by using the formula and plugging in the numbers and then solving.