Help!: Tendonitis at 2 weeks before 20k run

I am inscribed for a 20 km run on Oct 14 (my first run!) and I have contracted a tendonitis on my right foot - my doctor tells me to go easy on training.

Before having my tendonitis, I was at 1h53 for 20km and about 52min for 10km - I haven't done any serious running for 10 days and I fear that I am losing my condition

Any suggestion on how best to prepare? I am doing some biking (stationary in the gym) ad weight training