I Am A White Male, 64yo. I Weigh 258#'s And

I am a white male, 64yo. I weigh 258#'s and have lost twenty pounds since I started bike riding. For five weeks I have been spinning indoors. I just started wearing a HR monitor while exercising. My max HR has only been 123 while exercising at a strenuous rate while standing up on the pedals for 90% of the 45 minute class. I take a Beta Blocker and monitor my BP at home each day. It is in the 120/80 range on the second test (always somewhat higher the first time) and my resting HR is 46-52 BPM. My quads tend to give up first while cycling or snow skiing. My recovery takes a day and I don't exercise the next day. Still, my performance seems limited considering my perceived effort and the quad soreness seems to get no better from one training episode to the next. My question is: is the BB limiting my fitness and is it indirectly, the source of my soreness in my leg muscles and their fatigue? It feels as if I cannot get enough oxygen to my muscles.