Actually, it has always seemed to me that, as a rough analogy, "renshuu" would be "practice", and "keiko" would be "training." At least in a sense of similar interactions between the words. But of course "training" doesn't carry all the nuance of "keiko"; the Japanese imported the word "toreiningu", so obviously they also feel that "keiko" and "training" are distinct.
And once we get into "training" (or even トレーニング) we have yet more terms such as kunren
修練 and tanren
鍛練. That's just what I hear in an aikido training/practice context.