I try to go on Tuesday, Wednesday and Thursday after work. It is 2.7 mi on mountain trails. (Picture dirt, roots, hills and creeks) I complete that in 30 minutes. My heart rate stays up the whole time. 30 minutes would be slow for a guy in good shape. Remember I am a girl. I am in great shape for a sheela but I can't compete with the muscle mass or lung capasity of a man.
On Saturday and Sunday I am out there at 7am. Now, if I am not pressed for time I am starting a 7 mi. trail that is more technical than the one I am used to. That'll get my heart pumping.
There is a huge difference between road biking and mountain biking. I don't know if you have done both but it is a whole different effort.