One thing I noticed is that I feel out of breath much before I feel tired in my legs, and when I check my heart rate it seems low. Like mid 160s, that seems low overall/absolutely, but also compared to my max (I have hit 175 cycling before, and from cross country I know it can get to the 180s and not too many years ago from running it would go to mid 180s or higher if I ended a run with a long sprint).
One difference between running and cycling is the ability to select the resistance or "cadence/load" . In running, your legs - and your weight - are both static. In cycling the exercise load present is a function of gear - inches (stride) over cadence, (stride frequency)