Should training be hard? Yes, but...
The word hard is defined as something, "solid, firm, and resistant to pressure; not easily broken, bent, or pierced" which implies that whatever it is is seemingly insurmountable. The word "hard" then brings with it a connotation that something is impossible or at least highly improbable. Aikido training teaches us that, although something is difficult, nothing is impossible. After a shift in one's perspective, training can move from being hard to being "challenging."
The word challenging is defined as, "testing one's abilities; demanding" and brings with it a sense that the situation is daring us to show our best selves. The training can then be a proving ground where we learn perseverance, determination, courage or any other seemingly hidden positive trait.
Both the words hard and challenging are adjectives and by definition adjectives modify or describe nouns or pronouns in order to qualify them. Therefore, they merely exist to bring about perspective to something.
Henry Ford said, "Whether you think you can, or you think you can't - you're right." If we perceive something as being hard and insurmountable then it will be hard and we won't overcome it. If we see something as a challenge to inspire our best, it will bring forth our greatness regardless of the outcome.
When I was a student, many of us would arrive well before class started to get in some extra practice and prepare ourselves for class. Sometimes we did this to tire ourselves out and make the class more challenging in order to as Ken Furuya Sensei would say, "to get the experience of what it would be like to do Aikido completely exhausted."
We made the training "harder" in order to challenge ourselves. We create the conditions for our own experiences and thus training is only as hard as a function of how we approach it. Don't get me wrong, Aikido training isn't easy per se, but it is only as hard as we make it. So yes, Aikido training is hard, but that depends entirely on us and how we approach our training. Sensei used to say, "Cry in the dojo, laugh on the battlefield." We can only do that if we challenge ourselves to be the best by training hard when we are in the dojo.