Is Fitness Required For Self-Defense?

The answer is a combination of the above.

Most attacks can be avoided if you understand the psychology of self-defense i.e. how attackers select victims, how not to be an easy target, how to recognize pre-attack indicators and how to de-escalate a volatile situation. 

Also, there are certain techniques that have a high percentage success rate at stopping most attackers or at least disrupting their attack enough to allow you to either get to a deadly force weapon or escape the situation. That is the reason why we say that anyone can learn self-defense… BUT

Not all conflict avoidance techniques are effective all the time. Neither are all physical self-defense techniques.  Even if you use a weapon, the attacker may try to take the weapon away from you and it may escalate into a prolonged battle for possession of the weapon. You weapon may also fail, thus forcing you into unarmed combat. Also you may not always have access to a deadly force weapon and you are forced to fight unarmed.

This means that a violent confrontation may escalate into a prolonged violent fight where physical endurance and the intent to cause injury  is needed to keep fighting back.  But the reality is  that you may have all the intent to cause injury but if you are too tired to lift your arm or leg, intent will simply not be enough.

Also, if you have to run away, you will need enough endurance to keep running for as far and as long as it takes. You may need to overcome barriers and obstacles such as fences, running through uneven terrain, down hill or uphill to get away. Will you be able to do that using the adrenalin spike caused by the attack? Most likely not. Remember that the attacker may chase after you. Simply believing that running away will end the attack is flawed thinking.

So, while it may be possible to defend yourself without requiring fitness, you stack the odds in your favor if you are fit.