The optimal amount of miles is the mileage that can be done sustain-ably without injury while still being able to do speed/stamina work and injury prevention work like strength training and stretching. Take your time resting up though, and when the injury has gone away take your time getting back into your 60 mile weeks, even if that means a couple of months of low mileage weeks. Talk to your coach on how to accomplish this too.
Believe it or not, not everyone can safely run more than 50-60 mile weeks. I'm not shaming mileage, but I'm saying if you notice that after your full recovery you consistently hit 60-65mpw and somehow get injured again that might be "God's way of telling you to back off". Speaking of that, make sure you're running your easy mileage days at a pace that allows for full blown conversation with a buddy, or yourself if you don't mind looking crazy. Taking the easy days too hard can cause muscular-skeletal fatigue from chronic under-recovery and make high mileage seem impossible when in reality you're running the easy days at something more like half-marathon/marathon race pace the entire run.
Mileage helps you become more neuro-muscularly efficient, but mileage alone isn't everything. Lots of elite runners, including Bekele, Bernard Lagat, Seb Coe, insert a plethora of 800m Champs, never ran more than 80-90mpw. As far as how to stay competitive, some runners are more talented/genetically gifted than others. It means that once you've recovered completely and you've finally found YOUR mileage, even if it's "just" 50-60 after months of chipping at it, you'll just have to be more aggressive in your hard workouts and races and be patient in your training.
Also, other forms of cardio like cycling, swimming, dancing real hard, elliptical, etc. can build your cardiovascular fitness too. So adding that into your regimen, given your doing your strength training and stretching, still fully recovering from workouts, and having access to that many calories to replace what you burn can possibly increase your aerobic fitness in a way that carries over into your running. Talk to your coach about supplemental training though.
Don't rush the training process. I know I keep saying it, but communicating with the coach regarding your training, even if it seems annoying at times, is key to healthy and intelligent approach to this crazy sport. That being said, I'm just a 20 year old boy on LetsRun that probably reads too much for his own good. Best of luck to your life endeavors BK. Thanks for reading.