I don't think the lowest group (D) should have to speed up or ride further to accommodate the better riders. Maybe the C Group need to break into two groups (C- and C+) -- it really seems important to maintain a group that doesn't overwhelm new riders. If you do end up speeding up the Ds, it may be necessary to add some E rides.

You are being a real sport leading the D riders. When I am first starting, it was wonderful to have better riders who were willing to show me the ropes.

I agree with Bob D that, with beginning riders, it's important to reward improvement as well as who's fastest. But I'm not at all opposed to the "first up the bastard hill wins" idea -- we all can use the motivation.

As your beautiful young lady becomes a stronger rider, maybe you can lead those "C-" rides and a new leader can take over the D rides. But I hope they don't disappear.