If you have hills, climb them with an effort that allows you to get to the top, but just barely. You should be panting uncontrollably after your HR comes up on the climb - otherwise ignore your HR and just concentrate on a panting level effort.. Recover back to normal breathing and do it again and again until you can't anymore. If you don't have hills, do the same thing except accelerate and stay at that panting level for maybe 5 minutes. Once a week. Once a week, on the flat, accelerate absolutely as hard as you can in a pretty big gear for 30 seconds, then 5 minutes easy, then repeat, maybe 10 times. Twice a week, ride for 2-3 hours on the flat or moderate hills, limiting your effort so that you could speak in full sentences or recite the alphabet the whole way. My guess would be 115 HR. As it is said, most folks don't ride either hard enough nor easy enough. Ride more at the two ends of the spectrum and less in the middle, as in this prescription.

If you want to ride with a group, ride with the one that's too fast. Just hang on as long as you can. If they drop you on a hill, keep riding hard as you go over the top to try to get them back. Once they lose you, just pedal the route at a comfortable pace. This is sort of the same prescription, but in a group scenario.