I use three methods of recording.
ViewRanger which uses OS maps
Garmin Fenix 5X which uses Open Street Maps - OSM
Fitbit
ViewRanger is the most accurate and the one I tend to use and enter data on a spreadsheet. I do the 1500 mile challenge and with the spreadsheet calculate average daily mileage, expected time to complete the 1500 miles, predicted miles at end of the year based on average, mileage with current boots and other useless information!
I only use my daily walks which average around 2,000+miles per year.
However, if I total everything I do, then it is closer to 3,500 miles per year.
The most accurate method is always to link to some form of good software maps such as OS in the UK. I find OSM the best when abroad.
Regards
Chris