I've been cycling avidly for going on 4 years now. When I first began, I made huge strides in fitness and endurance in very short order. However, over the past two years I feel like I haven't made any noticable improvement in fitness, endurance, strength, or any other marker that I can think of.
Some background. I'm 37, 5'10", 163lbs. Approx 15% bodyfat. Resting heart rate approx 60bpm. I ride 4-5 days a week. Some days I'll ride alone at my own pace, generally riding distances in the mid 20's, average MPH generally hovers around 16mph. Several nights a week I hook up with local group rides, and I *try* to keep up with the fast group. I'll be able to hang on for a few miles, but I eventually get spit out the back. I do this with the hope that stressing my body will allow me to hang with them for progressively longer periods of time. But try as I might, I just don't seem to be making progress. I burn out waaay too quickly, and can never hang with them more than 5 or so miles of the 20+ mile routes they take.
Also, I would think that by now my heart rate during exercise would be lower on average than it was three years ago. It isn't. Any effort on the bike ramps my BPM to 150-160, a moderate effort kicks it into the high 160's thru mid 170's, and anything strenuous pushes it to the 180+ mark. This is no different than it was 3 years ago. This would lead me to believe that my cardio fitness isn't improving, and it's both frustrating and a cause for concern. My recovery rate however has improved somewhat. If I stop and rest after a 180+ bpm effort, by heart rate will drop into the 140s/150s in approximately one minute.
Any suggestions on what I can do to force my body to improve? It's humbling to get my ***** handed to me by retirees, and I'd like to stop the humiliation