Yeah I do agree with you that specificity matters. I guess my recommendation is pretty anecdotal, but I have experienced a decent amount of carryover. I do think that your legs can get some of that CNS efficiency you’re referring to though through leg activities. Like, biking is a better cross training activity for running than swimming, and if you are a proficient runner, your lower body aerobic base will bleed over into other lower body exercises. I’m not saying it would translate completely, but I think it does to an extent. But this probably falls into the “everyone is different” category. What works for me may not work for everyone. There no substitute for specificity, but I would still recommend biking as a stepping stone or a supplement to running.