Training two days a week isn't enough exercise for me.
This is also a really good point. Good fitness does equate to good Aikido, pretty much true for any physical activity. Fitness like anything is defined by your circumstance and is relative but must be taken into consideration.
This is different from the original poster asking about fitness training before starting Aikido but if you are only doing aikido twice a week - even the aikido specific conditioning will not get you anywhere.
Supplementing your training with other off-day exercises is a good idea.