A 49 year old shouldn't need to dial it back, if you're in otherwise good health. I moved to Colorado at age 50 and went from the relatively flat terrain around the Kansas City area, at 800' elevation to living at 6,000' and riding on up to 14,000' Mt. Evans. My average speed dropped about 2 mph, between all of the climbing and the elevation, but I never had any different effects from 3 hours of hard riding. I rarely rode more than every other day (50-60 miles) and I only rode that much because I wasn't working. It took several years for my climbing ability to improve, but at age 53, I rode my best time from Idaho Springs to the top of Mt. Evans 2:35, that would often be a top 10 finish in my age group, on race day. After than, I never got any faster and old age was catching up with me. I quit cycling at the end of the 2010, at age 57 season because my knees were shot and I had a house to build. I got my knees replaced in 2017 and took up cycling again in 2018 at age 65. I'm only riding 3500-4000 miles per year instead of 5000+, but I'm still riding and enjoying it. If I ride 2.5-3 hours every other day, I'm satisfied. I use a HRM, just to judge how hard I'm working, but I have no goals other than to make it up the 12% grades.