Firstly, I understand that different runners react in different ways to training, and so a specific case cannot be made to each runner.
When starting out running, there is a lot to be gained from incrementally increasing weekly mileage, eg going from 10 miles a week, to 20 miles a week. Generally, the improvement is brought about from the increase in cardio fitness, as well as the additional strength built up from the increased mileage. However, there must be a point at which the gains are very slight.
Professional, or Olympic athletes, are known to train over 100 miles/week (I believe I read somewhere that Mo Farah's average week is ~120-130 miles). The top-end of non-professional athletes often train towards to 100 miles/week mark (although that must be tricky balancing work and training!).
My question is; at what point does the performance improvement, brought about by the increased mileage, become marginal? Considering 3 popular race distances, 10km, Half Marathon and Marathon, and assuming an 80/20 (easy/hard effort) training load, with an easy long run taking ~30% of the weekly mileage.
And, for context;
As a 22-year old male, currently sitting just below 70% on the age-graded times for 5km, 10mi and Half Marathon, (18:28, 66:38, and 1:28:14 PBs respectively [no 10km raced recently]) I am considering upping my weekly training load from ~50km (31mi) to about 50mi over the course of 3 or so months, in order to attempt to head towards the 80% age-grade times for 5km, 10km and HM (16:08, 33:35, and 1:14:04, respectively). The HM PB was the result of a good 12-week training period over the summer (50km/week during and for ~6 weeks after), and the 10mi PB hit in November, however I broke my collarbone shortly after, and am restarting the training again.