Crosstraining during aerobic capacity building

Hi, I’ve been working on aerobic capacity building due to my ADS (primarily for ultrarunning) and am wondering about crosstraining. From everything I gather from the UA resources, maximizing the training volume at Z2 will maximize the rate of increase in the AeT (injury risk notwithstanding). However I’m trying to avoid more than 8-9 hours a week of training running on foot to avoid risking injury, so I am wondering how effective crosstraining is in terms of building the aerobic base?

More specifically, I would like to do an additional 4-6 hours of biking/skiing/erging in Z2/week if that will help build my base, but I’m not sure if these are less useful than running. Is aerobic capacity a function that is improved locally (hams, quads, calves…) when stressed via AeT training such that crosstraining is less effective, or is the benefit systematic enough that crosstraining in these forms will help increase my AeT?

Thanks!
PK