My mother, who is 60 years old and never exercised consistently or intensely in her life, is always telling me I ride too far and too frequently. I go for a quick 15 mile ride after work and she tells me I am going to harm my heart. 15 miles to her sounds like 15,000 miles.
People who don't ride bikes just don't understand how easily you can put some miles behind you. I took my 7 year old son for ride on Saturday and he rode 18 miles and wasn't even slightly wore out. Biking is as intense as you want to make it.