How many kilometers can I run by mountain bike every day to achieve the purpose of fitness?

This problem cannot be generalized, and it has a lot to do with age, gender and speed:

Young people and middle-aged people (male): Generally, they ride 20 kilometers at a speed of about 20 yards.

Female: In general, she rides 15 kilometers at a speed of about 15 yards.

Older people (male) over 55 years old: riding 15km at a speed of about 10- 15 yards per hour.

Riding, under safe conditions, with such speed and mileage, can achieve better exercise effect.