I would recommend a ride where you try a little bit harder once a week.

Maybe even join the B group for a ride and just hold on as long as you can.
Then each week make your goal to hold on 5 minutes longer, or 1/2 a mile or a mile longer...

Riding with others slightly faster than you can assist you in your goals - as long as you realise that initially your might get dropped (this is where I am in my grade, but last week I was wrapped because instead of being dropped at my usual 5kms out, I hung on til 10kms... I was thrilled even though I had to ride the remaing 29 kms on my own!

Ride often, ride challenges you set yourself.

I also suggest you choose a course you ride often, and measure yourself on it every 3-5 weeks. Ride it as fast as you can each time, and slowly but surely your should begin to see improvements.

Remember, you are asking your body to make a metabolic change - for your heart to respond differently to stress - this takes time so be patient with yourself