You can add repeating events in Calendar but the “race” part of the dialog (which is the same as that used in Plan Builder) doesn’t have the repeat option so it would just be “rides” that lets you do that. You can’t add “rides” to Plan Builder, I suppose you could just make the Saturday workout an outdoor one.
I don’t think Plan Builder adjusts the subsequent workout - just tried with Medium Volume and playing around with duration and intensity of a Saturday event and it suggests the same workout regardless.