There was a study put out nearly a quarter a century ago on the virtues (or lack thereof) of supplemental training to aid in sports, in this case, swimming. At the time, when I was swimming intensively (6 days per week, nearly 25,000 meters per day), it was the custom to do weights three mornings per week, with additional training mandated by major events, such as "nationals." The gist of this study, put out by, if memory serves, a professor out of Cal. State Bakersfield, was that it is nearly impossible to aid a given sport by training in other than that sport. In other words, because a given sport utilizes whole-system resources, by specific muscles, specific parts of muscles, at a given rate of firing and duration, it is impossible to aid those same muscles and therefore improve upon one's system conditioning by doing other than that sport. To wit. I was a distance swimmer. When I was sick, I would stay out of the water but still train, by doing, for example, high-rep triceps extensions, etc. The problem is that the rate of the swimming motion, angle of attack, etc., were not possible to replicate and, by this study, this training would offer very little towards my swimming. What it did help with was my ability to lift these weights, in this manner.
So, if I understood correctly then (and memory serves well now), if you want to gain aerobically by doing Aikido, tax yourself aerobically by doing Aikido - don't ride a bike 3 x per week at 100 miles; etc.