What gear are you riding in? Your cadence needs to be 85 to 95 and drop into whatever gear lets you do this without exhausting yourself. I'm 50, weigh 200, ride 6 out of 7 days. One ride over 100kms, 50kms x2 and 25 x3 each week. I take one day off. My average rpm is 88 on all rides, keep my heart rate around 135, average speed is 28 to 30 kms/hr. Try to push yourself on each ride but for short durations. The more you ride the easier it becomes.