If you're coming to cycling from the world of running, the rule of thumb that I use is 7 miles of (road bike) cycling is roughly equivalent to 1 mile of running (conversion to metric is left as an exercise for the reader). And, in general, require about twice as much time. It's a different kind of work out than running. Cyclists don't "hit the wall," we "bonk." And that only happens if we fail to eat and drink enough during a long ride. Also, it's not nearly as punishing, physically, in my opinion. On the other hand, runners don't get saddle sores.
If you're already a runner, you're probably in fine cardio shape, but allow some time and practice for leg and lower back muscles to come around.