At 50-60 miles per week, you can still improve by adding some mileage, and if you can add doubles, whenever you can, to increase mileage you are likely to experience increases in endurance over time. I don't think your first priority is to fit doubles into a hard schedule - you work 10 hours per day. If I were your coach, I'd tell you to increase your mileage on your non-work days, which is 3 days per week. Add 2 miles per day to each of your non-work days and that 6 miles more. Do that for a month, adding the miles at a slow pace. After a month, add another mile to each day. That's now a total of 9 extra miles per week. Run that for a month and see how you feel. If you feel good, add another mile on each of those non-working days. Now you are 12 miles a week more than you were when you started three months earlier.
In my experience, every 20% increase in training volume results in noticeable improvements in performance. Of course, there is a limit for each person, but you won't know what that is until you actually try to reach your limit or surpass your limit.
Another point I'd like to make is this: You may become fitter and stronger and eventually faster in races by running more miles, but most of the miles you add are slow paced. If you saw how slow Andrew Duncan runs, a 44 year old guy can ran near 2 flat in the 800, 4:05 or under for the 1500m, 8:49 for 3000m, low 15's for 5000m, upper 31's for 10km and he just ran a 1:11:00.0 for the half-marathon (he won the masters division in Las Vegas, and it was his first long race...above 10km.), you'd be amazed. He often runs over 8 minutes a mile, but running slowly allows him to run fairly high mileage, week after week, year after year.
Paul Peterson, who posts here at The Run Zone has run 2:18 in the marathon. Read his comments about mileage. When he runs around 60 miles per week a good amount of his mileage is close to 6 minutes, but when he prepares for the marathon and his mileage ins 90 to 100 per week, he's running a lot of his mileage about 7 minutes per mile. He SLOWS DOWN when he runs more mileage, just like Andrew does. When Andrew was first coached by me he was running 40-45 miles per week and at a strong pace (under 7 minutes a mile) and he was at 16:30's for 5km at age 37. IN the last two years, as a 43-44 year old, he is running more than a minute faster over 5km and he's running near 80 miles per week, every week except a peak week. Most of his mileage is 7:45 to 8:20 pace, a minute to a minute and a half a mile slower than when he was running 45 miles per week, younger, and slower (over a minute slower in the 5km event, back then, compared to now).
In my view, it all boils down to about 1 hour of running per day. Once you go above that amount, you better run the additional mileage easy. My belief is that you'll last longer, improve more, and be faster in races once your body adjusts. So, my rule of thumb don't run mileage that exceeds 1 hour per day faster. This isn't referring to individual (key) workouts that are supposed to be faster, but it refers to all the extra mileage you add to run between key workouts. Think about my rule of thumb and figure out if it makes sense to you. Perhaps it can guide your efforts to increase training volume effectively and it may result in better racing.