It sounds like you are on the bike a few days a week. How long are the outdoor rides? How long have you been on the current plan? What is your physical background? And how much time do you have available or want to invest are some of the questions that folks may ask.
There is a great thread about someone who measurably increased their cruising speed by several mph (I'm guessing from the mid teens to low 20's) in just a few months if you are willing to do some reading: http://www.cyclingforums.com/t/492377/new-guy-with-140ish-ftp-looking-to-hit-225-by-may-2013
(You don't need a power meter to grab the jist of the recommended workouts and the path the fella took. And if not familiar already looking up general terms you aren't familiar with will also help with future advice)
In a nutshell - after a brief warmup, a workout of a couple of longer intervals (20 minutes or more at about 85-90% of the effort required for the max cruising speed you could sustain for a hour or so) are recommended. It would be an intensity slightly above what is defined as "tempo" (where only short sentences would be possible if tried, and the effort is tough enough to require concentration to maintain, but obviously a level of intensity one could hold without petering out before the 20 minutes is up). Then 10 minutes of easier pedaling in between to rest. If you can't hold the effort steadily for both of the 20 minute efforts you went a bit hard and need to dial it back a bit the next time.
This workout can be done a couple times each week but ideally should be tackled when adequately rested, and in your case sounds like it could easily replace one or two of your outdoor rides (the workout can be done indoors or out but if outside it's best to have a circuit or stretch of road where the effort can be done uninterrupted). Longer and slightly less intense intervals give great bang for the buck (i.e. invested time) and aren't as likely to burn one out (either physically or mentally) over the long haul.
One thing important to understand is that different workout intensities all focus on building fitness but in slightly different ways. Different intensities target different energy pathways (each with their own associated physiological adaptations, some of which overlap between workout intensity levels) and some have greater potential to develop over cumulative years of training than others. The nice thing is the benefits of the workout described above can be developed and stacked cumulatively year after year. Frequency, consistency, and quality of workouts along with adequate rest are the key to getting faster on the bike.
Welcome to the forum.
