It would be against my better judgement to accept your use of only the rear brake. Look at fixies that have brakes. They often choose not to install any brakes, but when they do, they (9 times out of 10) install front brakes on their bikes. The front brake has the most stopping power. If you plan on stopping from a speed above 10mph with just a rear brake, plan to have 100ft+ to stop and to replace your rear tire every 150 miles. Run with just the front brake and force yourself to learn to modulate brake power. To be a good cyclist (or a safe one, for that matter), you need to learn how to modulate front brake power.