Marathon training interrupted and only 3 weeks to go

I've been running for 2 years, and I plan to run the Dublin City Marathon in 3 weeks time, but have been out with peroneal tendonitis for the last 16 days. My training plan is the Hal Higdon Novice 1. I've completed all training up to and including a 18 miler just over two weeks ago, with ease and no issues up to that point. but have not run since due to the injury. I am recovering, albeit slowly. I've run a half marathon in 1.45 with relative comfort, and was shooting for 4 hours for the 26.2, but it's a given that I will need to revise that goal now. I've done no alternative training, but plan to cycle 30-40 K this week over 2-3 days. Assuming I recover with 2 weeks to go before the marathon (i.e. having lost three weeks of training (including my 20 miler)), what should I do with training? Rejoin the program mid-taper? or something else? Should I actually just hang my hat on this and refocus on a spring marathon? I'm completely dispirited by this turn of events, and am quite unsure what to do here. Any help would be much appreciated. Also, what spring marathons are there in the country (if that be the advice)?