Cycling, quick Q not too important

[MATT15]

First of all just to ask, what is the general opinion of cycling as a method of cardio. i am talking about a bycyle that gets you places as apposed to a stationary fitness machine. I am in pretty good cardio shape, BUT my ankles kill me when i run, its really bad. i used to be able to run for an hour and not be out of breath badly. Now i cant do 15 because my ankle kills me. so i thought id give cycling another go. If this does turn out to be a choice, do you all think the gears will have an effect? should i let my legs spin easily but a hell of a lot more on a low gear or a really tough time but less revolutions???

Might seem pretty neglible but the faster i can cut the faster i can get back to bulking so whatever helpzs