There are many reasons to increase weekly running amount, for example when you have been training for a while and need to new challenge and develop your running fitness or start training for marathon. Often during marathon preparation program mileage/kilometers amount increases gradually, this increase depends about your fitness level and how long your training program is.
There are different ways and method to do this. And for sure you can also mess this process totally. Too fast increase may lead to overload and injuries, which no one want.
I won’t describe any certain amounts and increasing strategies as these are independent and vary so much your current mileage and training style. You should also take account that it’s not suitable to start to increase mileage when you are about to run any race in the near future or when you are develop speed and main training is focused power, or it will be very difficult as you then want to develop two different factors at the same time. Also if you have been start running a little while ago or coming back after injury or break, let your body time to adapt running.
First you have to know your baseline of training, how much your current normal weekly amount of training is this can be seen when you calculate average of every week for past couple months. Do not take account if you have run any race and there are tapering and recovery weeks. Think how much you can adapt more training, if you are busy and used to run 50km per week there’s not point to try increase your training 150km per week, as you would not have time for that and your body could not take that amount in near future.
If you are running 3-4 days per week add one more run training for week. You can cope this style also if you are running 5-6 times per week, if you are want to one or two rest day during the week, this means you have to train twice a day at least once a week. This might mean your weekly mileage increase 15%-20%, which is often too much. Experienced runners, whose run more can increase about 10% per week and again beginners and runners who don’t run as much might add one short run more to the week or extend one of their run.
Don’t increase your running amount every week, depending on fitness level and number of runs/kilometers body need at least couple months to adapt new training amount. Take easy week every 3-4 week. Once you are very familiar and used to run more you can increase your running amount more, but don’t make too many increase during the year as you might have to train speed and other elements also or prepare to races.