Why are you training? What are you training for? I find that a concrete goal is the first step to beginning a training plan. You say you just don't want to be the slowest rider in your group - that may not be specific, achievable, etc. enough to be a real goal. How about finding a ride you want to do - a century or metric - and use that as your inspiration. Or if it really is just "be faster", how about a goal of doing a usual ride in a specific time - and then work on that?