Riding without a rear brake really is not a major issue. It would be, however, a major issue if you rode without the front brake.

Reliance on the rear brake is so much witchcraft. It is a little effective -- by about one-third compared with the front brake. This episode may help you understand that your front brake is your primary brake.

Ride your bike pretty well as you would normally, but ignore the rear lever. Don't "grab" the front brake lever, but squeeze it progressively. Oh, and keep your elbows straight and locked if you have to brake suddenty and hard to stop you going over the handelbars.

When you get a chance, take the bike to your LBS to get the spring put back in or replaced.