You have many ways of recording your mileage while riding outdoors. You can either take your mileage off your cycle computer such as a Garmin and Wahoo and then connect these up to a Free Strava account for your own personal training log.
You can also record your rides through the Free Strava app you can download on your smart phone and then press the record button to track your ride and this will be saved on your own personal account.
Another way is when you get home from your ride you can go on a free website called ridewithgps.com , then map out your ride on there and take the distance off there and type it into a spreadsheet for the month.