Just choose one... say, you Wednesday ride. Tell yourself something like... "Every Wednesday, during my ride I will ride as hard as I can in the biggest gear I can for 2 minutes, then I will rest and then I will do it one more time."
That's all... after a couple of Wednesdays you will do your two intervals and think - "Hey, I still feel fine, I'll do another one or two..." Voilah... without realising it you will have baby-stepped yourself into a cruise interval training ride.
Or maybe Wednesday doesn't work for you, and you don't want to work on surging and keeping up with the group. You get dropped on hills. And, hello, on Tuesdays you happen to have a steady climb in the route you normally ride that day. OK... ride as per normal, but when you get to the top, turn around, go to the bottom of the hill and climb it a second time. Try and do the second climb as quickly as you did the first - or more quickly!
Voilah again... in a few weeks Tuesdays has become a specific hill rep day to make you stronger, give you more endurance and ensure you are less likely to get left behind on hills...