Cycling improves aerobic capacity. Don't use it as a main form of training because it's a different muscle group. It's a good alternate workout, though.
Also, you have to go a long distance and make sure you're doing it right to get a result. In the beginning, you may not be able to do 25 miles (which is considered a short ride), but distance improves with practice. Do what you can at first, just do it right and give it a good effort.
