I have always found it takes me a long time to warm up and get "in my zone" when exercising. If I'm on the treadmill or taking a class or out for a bike ride, I don't feel like my body is "into it" until about 40 mins into the workout... which is usually over after 60 mins if I'm at the gym. If I'm out on a group ride, it's hard to keep up until I'm thoroughly warmed up. I am 52, have a med issue that doesn't prevent me from exercising, but can/does effect energy level, so I know this may have some impact. In the past few years I also see it takes me longer to recover after a hard workout & I might need a day off (even 2!) in between challenging days.