HI Karen - I would definitely am for a 50k as a training race and ideally this would be no closer to the 100k than about 5-6 weeks before. The half marathon would be a good replacement for a short hard training run and ideally try to make it on similar terrain challenges as your 100K. You could probably squeeze two half's in there if you wanted.
I also got your email and will respond to that with comments on Training Plans.
Let the training Begin.
Hi Karen, It's really very nice to hear from you about your preparation. I think 50 km can be an ideal race for you. I have captured some cool ideas for training from "natrun.com.au". I hope your training program will go on a perfect way. Thank you.