You're right that as a runner, your lungs are probably ready to rock. However, the leg muscles involved in runny vs cycling are radically different and it'll take time to build the cycling-specific leg muscles.

I think the advice regarding resistance and cadence is good advice. If you start training with a power meter, you'll learn there are two ways to generate power: mash your pedals at low RPMs or spin your pedals at high RPMs (or a combination of both). I personally prefer the latter... kind of like how race cars (Indy, Formula 1, etc.) are crazy fast but also run their engines at crazy-high RPMs. The benefits of spinning are that it taxes your lungs more, which is the thing you already have well developed. Granted, this won't grow huge quads, but in the long run I think it's better for the health of your legs... more specifically, the health of your knees. I was warned when I was in my teens that I shouldn't mash gears or else I'd risk wrecking my knees when I got older... that advice came from a ex-Olympic cyclist. He was right... I'm now 46 years old and my knees like to talk back to me :-) While I do enjoy an occasional all-out sprint, I'm a firm believer in "spin it to win it"!