When building up running mileage, it’s important to do it gradually to allow tendons, ligaments and muscles to recover before stressing them with the next run.
The safest way is to not increase running mileage or time by more than 10% per week. A three week build cycle followed by one week of recovery is a sound approach.
I go by time run instead of mileage because it’s easier to calculate. Simply time each run and add up the total number of minutes spent running in a given week. For example, if you went for a 30 minute run on Monday, a 60 minute run on Wednesday, a 45 minute run on Friday and an 80 minute run on Saturday, your total for the week would be: 195 minutes. Increasing by no more than 10%, that means the following week should not exceed 214.5 minutes of running (195 x 1.1 = 214.5).
For three weeks you build, increasing total time run by 10%. The fourth week is a recovery week; therefore you run half the average of the previous three weeks (195 + 214.5 + 236 = 645.5 / 3 = 215 x 0.5 = 107.5).
The fifth week is the same as week two. Build in the same pattern.
Week 1: 195
Week 2: 214:30
Week 3: 236
Week 4: 107:30
Week 5: 214:30
The number one reason for running-related injuries is simply doing too much too soon. This is the best measure that can be taken to protect against that.