I may not be the best to chime in, but with 3 weeks remaining (2 to 2.5 training if you taper) you may be down to the wire. All the training you have done to this point will be what carries you through. If you do the 100 mile ride you may want to do that within the next week or so. I like throwing in the 100 mile rides because it gave me confidence that I can cope with the time in the saddle. So for me it is more of a mental aspect than the physical aspect.
You have to manage what you do in the next couple of weeks and determine the accumulated fatigue from the previous weeks of training. Even if you recover in a day from the metric route there is still accumulated fatigue that may exist, but it is hard to sense. You will know it when you do a longer event and feel it.
Yesterday I felt it from training all week and then trying to hang with my group on a rolling hill route. My legs were not sore and I felt good before the ride start, but after 30 miles or so of repeated hills and them hitting 350+ watts on each roller I could then tell how fatigued my legs were. It seems that you are asking the right question if doing 100 will wear you out too much, but maybe you are the only one that can really answer. If you had weeks of accumulated effort in WKO PMC others could probably look at the chart and give some refined advice.
If you have a power meter it is even better because you can use that to gauge your intensity level during the event. I use mine to keep my focus when other riders go by me as a reminder not to burn matches that will cost me later. I find it real tempting at times to jump in with a fast group rolling by in an event, but if their pace pulls me higher I know I will struggle later.
Best wishes and let us know how it goes. It is always great to hear how folks do in the event.
