Thanks for the responses. Sorry for the lack of info. The first three rides were about 12 to 13 miles. The second ride was about 10 miles on pavement with about 2 miles on MTB trails. My average speed, according to app, was about 9 to 10 mph. This may not sound like much but in the last year or so my level of cardio excercise has been pretty low. The extent of my exercising, prior to getting MTB, was push-ups, air squats and light weight training. There was no shortness of breath or anything like that today. Just low energy. At peak times my heart rate averaged about 160 BPM. Thanks again for y'alls kindness.