Explanation

Check the Graph Editor for the foots forward/back movement by scrubbing the timeline. Whenever the foot is planted on the ground the tangents should be set to linear.You should have a straight line graph from the point where the ball of the foot first touches the ground to where the foot picks up.
At your contact position, note the z translate of your characters foot (or x and z). ie. it starts at 4.00 in the contact position and ends at -4.00 before it lifts off. Then the length of the stride is 8.

Method

To elimate the slip, Select the global control and open the Graph Editor and then set a start key on the translate z at frame 1 and the end key at the end of your walk cycle.
Select both keys in the Graph Editor and set their interpolation to Linear (that's the 3rd tangent icon that looks like an upside down v) then under curves > Post Infinity click on Cycle with Offset.
Scrub to the end foot planted position and then select the end key in the foot down position and slide the key in the z up to around 8.and check with a scrub to see if there is slip and adjust + or minus a little until the slip is eliminated.