Jean, my sympathes - I know how frustrating it is. I'd say it's really going to depend on how weak your ankle is after six weeks in plaster, how quickly your body allows you to ramp up the training miles without developing overuse injurues, and how slow you're prepared to go on the day. You're going to have 17 weeks from plaster off to target date, which is pushing it (I do know some people who can jump from almost no running to 40 miles a week in a couple of weeks, but they're definitely exceptional!). You may find some run-walk schedules which would get you round.
Alternatively, you might find it safer and in the longer term more satisfying to look for a HM to run in June on your way to an autumn marathon. Good luck!