Alternate periods of gradually building up mileage (increasing weekly volume by roughly 10%) with periods of gradually building up intensity (increasing workout volume or workout pace). The off-season is a good opportunity to spend periods of 4-6 weeks working on increasing volume, for example, increasing from 20mpw to 35mpw over a 6 week period. In-season is a good opportunity to work on increasing workout intensity, for example improving from doing something like 6x400m at mile pace to 10x400m at goal mile pace. Off-season workout should be Tempo workouts, progressive long runs, and some strides, short hill reps and maybe some 200m or 300m repeats every few weeks to keep a bit of speed in the legs year round. In season workouts should still include some Tempos but can move moreso to some race specific sessions such as the 400m workouts previously mentioned. Best of luck with training!