For a 2D animation you would probably plan every movement and divide it up into 'key' positions, and then plan all the in-between drawings. Clay animators don't work this way. Once you get started you need to go on until you are finished.
Walk or run cycle varies depending upon the mood of your character, the environment and the situation
Have you noticed while running there is a point when both your legs are in air; to animate such action we use a stand to hold the character.
If you study walk cycles and run cycles thoroughly you will notice that movements originate from the centre of the body - the pelvis. The legs should be pulled forward from the hip area and adjust the head and shoulders accordingly, so as to balance the pose and distribute the weight equally.
Initially you may not feel confident enough for creating the movement; in this case you can always refer to movement from real life. But, from now on you can also be very observant of all the activities that are happening before you. If your friend suddenly falls down the staircase, memorize his actions and positions in your head, while you give him a hand to help him up.