1. Make sure you have a known distance to walk. Could be 400 meters to walk four times or a measured one mile distance.
2. Walk as fast as you can without feeling strained.
3. Note the time and the heart rate when finishing the distance.
Walking at given speed requires a certain oxygen uptake. If heart rate is known together with age and gender it is possible to estimate maximal heart rate and thereby maximal oxygen uptake.
Estimation of VO2max from a one-mile track walk, gender, age, and body weight.
Kline GM, Porcari JP, Hintermeister R, Freedson PS, Ward A, McCarron RF, Ross J, Rippe JM.
Med Sci Sports Exerc 1987 Jun;19(3):253-9