1 Just map out your walk or Run...solve for distance.
2 Wear a watch (any will do) and time your walk.
3 Check the time you start walking
5 Check the time you finish walking/running
If your run/walk is 5k. and you have completed in it 30 minutes...well then you walked/ran a 6 minute kilometer.
30minutes/5k=6minutes per kilometer
there you have it
try Runstoppable Running Log
for mapping your run if google does not work as suggested.