Easy from this point, without obstacles the rest of the way. For example, Once we had the basic design, it was downhill all the way.
Deteriorating or declining from this point on, as in When the cancer couldn't be removed, it was downhill all the way for him. The usage therefore needs to be clarified by the context to indicate which of these opposite meaning is intended. The figurative use of downhill dates from the late 1500s. Also see go downhill