I have a gps watch that measures my pace. Though you can also use a number of websites such as map my run which will calculate your pace based upon entering your route and time. Did a long slow run this morning at a much slower pace than usual. Did 8.97km in 48.49 didn't push myself at all could have run another 3K but didn't want to over do it as I felt a twinge in my right knee.
Main reason for doing the long runs so often beyond improving lungs and heart is for the cut down in bodyfat it does. If you looking for staying within a specific weight category.