My thinking based on your post:
You’ve 3 weeks to race day, we’ve written off one for a cold/recovery time, leaving you two weeks of training including taper.
Last week before the event is, as you say, a couple of easy runs with strides to open up the legs in the week, with the previous weekend including a tem mile effort, (at race pace I guess).
Two weeks out is normal training with the long run of 15 miles at 7:30 pace.
I see little advantage of increasing the 15 mile run to 20, unless you’ve either a good base and know your body would recover, or you’ve never gone that long before and need the mental boost.