I just ran last night for 105 minutes. I don’t have a bike where I am living and I wont’ have one for the next few months, so I run XC again. I’m 5’8", 140 lbs, 26 yrs old. My heart rate stayed between 165 and 170 and that is pretty typical for a LSD. During long races I can keep it above 175 if I have good training and nutrition behind me… but that’s running.
On the mountain bike I can’t keep anything steady due to all the climbs and downhills. I can go from above 200 for a couple minutes to below 150, just ebb and flow all day.
I agree with bsheidler in that you’re going to burn more fat overall at something above 125 BPM. I don’t even start workouts until I am warmed up (signaled by my HR being above 145).
Whatever the case, enjoy the ride.
R