High intensity interval training and weight training has completely transformed my body since I started Aikido about 9 months ago.
When I don't slack off on my HIIT training, my endurance during randori is practically a cakewalk. Last week I did randori after skipping the HIIT for a number of weeks and I thought I was going to die.
Weight training, both lower and upper body, has given me the ability to take much better ukemi. Even when I mess up a high flying breakfall my muscle devlopment prevents injury and helps lessen the impact.
I have not lost any flexibility either. In fact, I continue to gain in flexibility so long as I stretch properly after lifting.
I am glad I listened to Kevin Wilbanks and others about this when I started because I can't imagine how horrible practice would be if I relied only on Aikido and distance running to get in shape.