Indoor training is very boring. Get an old used mountain bike and find some trails in your area. You can avoid riding in traffic and get a good workout. Riding in the winter is fun as long as you dress in layers and practice a bit.

If you are immune to being bored, many high schools and middle schools have quarter mile tracks that are open to the public for jogging and running. As long as you are polite and stay FAR away from joggers, you can ride your bike on those tracks. Because there is no motor traffic to deal with, the only limit on how fast you ride on a track is your legs and your lungs. I ride on a track from time to time...but it is astoundingly boring.