Measuring with a heart rate monitor I burn similar calories in 60 minutes of brisk walking (6km/h) as I do in about 40 minutes of cycling at 20km/h on my comfort hybrid bike.
Walking an hour I cover 6km, riding for the same hour I cover 20km but am using 33.3% more energy to do so, if we take that off I guess in my case 6 walking km = 13.333 km or 1km walk = 2.2 km ride
However, I walk fast and ride slow, your mileage may vary (pun intended)