I know that many programs for beginners, especially in running, tell you to focus on time. However, I didn't do that. I mapped out some shorter routes (12-15 miles) that I could do when I got home from work. Doing those short rides 2-3 times a week, plus one-two longer weekend rides got me going fairly quickly.
Since you have a goal of doing 20-30 mile rides, I would focus on building mileage. The general rule is add 5-10% a week. Once you get comfortable with 25 mile rides, you can gradually focus on your speed.
Just my opinion.