i wish i burned 131 calories for every mile i walked! The last time i wore my HR monitor it said 20 calories for a mile walked i think.
It is hard to compare the two as there is a lot of possibility for variation : if you are walking 'fast' it burns 300 per hour, if you are cycling 'slow' it burns 300 per hour. But who decides what fast and slow is? I suggest you get a HR monitor, you input your height,age and weight and it determines your caloric expenditure based on these factors, and using your HR.
As a very loose guideline, i would say if you are exercising a good level of intensity as you walk/cycle, that one hour of walking and one half hour
of cycling might be about equal.
i don't think there are many people who walk at 5mph or cycle at 5mph.