You are in good cardio shape from running, and your legs are strong. However, you will be using your muscles differently when you are riding. So, I would not recommend pushing yourself too hard at first. Go out and ride as much as you can, but keep the pace only moderately difficult, and try to increase your mileage gradually. This way, you will build up a solid base, and get your muscles ready for some more effort. Then, you can try increasing your speed. Some sprint intervals (on the road or on the trainer) and/or hills will help you build strength/power.

Basically, take it easy at first, have fun, and build to where you want to be. It won't happen over night, but in a few months . . .