How much easier is cycling compared to walking or running?
I know calorie counts vary quite a bit, but a rough ballpark for cycling at a 20 mph pace is 450 calories per 15 minutes according to this calorie calculator.

Running a 6 minute mile is expected to burn a similar 450 calorie count on the same calculator with the same settings.

So, if you do the math, and the calculator is assumed to be reasonably accurate, then 5 miles on a bike at 20mph, is roughly equivalent to 1 mile run at a very fast pace.

Running a half marathon, which is 13 miles, and ignoring any difference due to endurance of effort, 13 miles at full pace running could be assumed to be a similar effort to 65 miles on a bike, in a very general sense. 

This is dependent on fitness, body weight, and many other factors.

The best method to work it out would be to measure power expended, but I don't know how to do that for running, and to be accurate, you'd need to do it yourself, and for the full distance of the half marathon.