If you are fitter, you'll be able to ride faster, yes. But terrain and weather conditions make a huge impact on how fast you can go. So where training is concernd it can be better to go on time, and how much effort you are making, than just assuming that fastest is best.

You are riding 5-6 days per week. You don't say how long those rides are. If they average an hour or more, that's pretty good. Let's say you can spend seven hours a week on the bike. I'd suggest a couple of rides of two hours each at your 12mph average, or whatever speed you can readily maintain for that time. That leaves three hours, which I'd divide between three rides. One would be an hour at the fastest speed you can maintain - not a sprint, but definitely going harder than is entirely comfortable, so you feel at the end of the hour that you have done all you can. A constant, steady effort. Another ride would be an interval session with five or six two-minute periods where you go as hard as you can, taking two or three minutes between each to recover. The remainder of the ride is a warm-up for 15 minutes at the start, and a warm-down for 15 minutes at the finish. And one day a week I'd spend an hour on the bike at a very gentle pace, deliberately keeping your effort level low and just spinning along really easily.

Take a couple of days off per week, especially since your job involves a lot of standing.

If you do this I am confident that you will soon be able to hang with your club mates on a 15mph ride. And when you do, you will find that maintaining that speed in a bunch is much easier than doing it alone, because the rider in front of you pushes the air out of your way.

Good luck. Let us know how you get on.