You've got your base of 500 easy miles (OK, you might have fudged a bit and it's only 300) but the days are getting longer and it's time to get serious about this cycling season.
Certain basic "rules of thumb" can be used to help you develop your own personal program for the coming summer, but the most important is not to push it. A little extra time and patience at this point can prevent an early injury while your muscles and ligaments are still a bit vulnerable. Using a training log is key to striking this balance. Both mileage and basic phsyiologic performance factors need to be monitored.
Calculate your average weekly mileage for the last 2 or 3 weeks, and plan to increase them by no more than 10 - 15% per week. This is a "softer" recommendation for calculating increases in your total weekly mileage, which may creep up towards 20% on occasion, especially the first few weeks of a training program as you try to make it worth getting on the bike for some of the shorter days. But you should use it as a firm guideline for your long ride of the week. (This example may do a better job of demonstrating the concept). The 10 to 15% figure was developed from marathon training to minimize musculoskeletal injuries. As bicycling is easier on the joints and muscles, the "fudge" to 20% is possible with only a minimal risk of injury.