Previously injured collegiate runner here. Took four months off due to a stress fracture in my foot. I have been back for almost three months now and have gotten back to my original mileage of 80MPW. I have just recently added in track workouts (I only did tempo and threshold work while building mileage), and I have noticed a huge decline in speed. I was previously a 3:49 1500m runner. Running 10x400 at 60s per 400 was my key workout last year and that wasn't too hard. I just did a workout where I finished with a few fast 400s and I struggled to run faster than 65. How long will this basic speed take to come back?