Mario and Sachamc, I love my big girl too much to hurt her. It will be up to her as to how far she car ride, hence why I drafted up the training regime. The other point is that it's not a race and energy levels will be no where near what a competitive cyclist would expend.
Like I said in my OP, she wants to do the GVBR with me so she needs to be able to do these distances. If we find in our training that it is too far for her to do alone on her bike, I will hook up the tag along so she can rest while I pedal. I think it's also a lot to do with "daddy time" and getting time to me all by herself and not with her two younger siblings.
I remember 2 christmas's back when I did my first 20k ride in 10 years how hard it was and how stuffed I was. The 65k ride I did 2 weeks later nearly killed me so I don't expect her to be able to ride 20k's or even 10k's straight away. We will build up.
On a side note, I'm really happy with her today. She's over her stack at Forrest on Mothers Day and has asked to go back. She didn't want to ride again that day or for a month or more afterwards but it seems her sense of adventure has kicked back in.
All valid points from you all and I will certainly take them on board and discuss it with her.