What about cadence? I always use a computer with cadence, so I know that I'm spinning and not mashing. I'm now 65 and probably rode 75,000 miles before my knees wore out in 2010, at age 57. I quit cycling and did other things for 8 years, but decided to try cycling again, after two knee replacments. My average speed has dropped about 2 mph, from what I could do 8 years ago. It's hard to find any flat roads where I ride. They are all either up hill enough to slow my pace by several mph, or downhill. I ride some extreme hills where I can hit 50 mph on the descent and climb as slow as 6-8 mph. On slight downhills, I keep my cadence at 90-110, and coast past that cadence in top gear. On the climbs, I try to stay in the 65-80 rpm range. Anything slower than that, means you should get out of the saddle or put lower gears on the bike. I use a compact crank and a 12-32 11 speed cassette, that allows me the choice to stand or sit on any hill I encounter. FWIW, at age 53, I rode the 28 mile race route from Idaho Springs to top of Mt. Evans in 2:35, which would usually get you in the top 10 of the 50+ age group, on race day. The fastest riders in that age group usually finish 10-20 minutes faster than I ever did. Although you ride to 14,000 foot elevation, that average slope is only around 4%. I never rode that route after age 53, because the average speed on my regular training route was always slower in subsequent years. I'd never beat my previous best time.

Past Results - Bob Cook Memorial Mount Evans Hill Climb

If you're not mashing a low cadence, then you're probably just working beyond your ability. Since I'm trying to get back in shape after so many years off, I take two rest days after every ride, rather than riding every other day, like I used to. I'm also not hammering it too hard, but I have increased my endurance to handle 40 mile rides on some challenging terrain. I used to ride 50-60 mile routes that include 10-15 miles of mountain climbing, but most mountain roads in Colorado are not as steep as the hills I now ride. My main goal now is just to stay in descent shape for my age.