There are multiple sites on the web discussing how to build the legs. Here is a link to just one site..http://strengthtraining.suite101.com/article.cfm/build_big_legs
Some trainers do recommend biking to build legs, especially uphill biking. You should not overdo cardio if you want to build muscle. After a certain time period the body will start breaking down muscle for energy. I believe the magic number is around 45 minutes.This is why long distance runners tend not to be muscular. Sprinters tend to be muscular with bigger legs. Hypertrophied muscle is necessary for bursts of speed. That is why the article recommended sprinting. Sprinting up a hill on a bike would be better than long distance riding.
You need balance in the diet also. Adequate protein is needed to build muscle.
Once you have more protein and carbs that are needed for metabolism; the excess will be converted to fat and not more muscle. A lot depends on your genetics if you will get fat or not. Some guys can eat continuously and not get fat, others guys gain fat easily. Genetics also play a role in how easy it is to build muscle and how much potential exists to be muscular.
Marathon Runner vs Sprinter