First off, don’t worry about your last 20 miler. It sounds like you can cover the distance, with 3 20 milers under your belt.
Right now you want to focus on 1) healing and 2) maintaining aerobic fitness without aggravating your injury.
As for healing, ice the crap out of it and take some ibuprofen to reduce the swelling.
For aerobic fitness, I believe Double has spent some time on the elliptical trainer before. There was a guy here 2 years ago who had a stress fracture but was an animal when it came to water running. That’d probably be my first choice, since it’s low impact, followed by biking.
See you in Boston. I’ll be the one with the replica Larry Bird jersey.