Marathon runners and other people involved in extreme endurance   sports like triathlons and long-distance bicycle races have been told that too much exercise can cause scaring in the heart.

(Researchers found that one in 10 athletes have signs of heart scarring.   Exercising for 30 to 60 minutes a day is the ideal, more can actually be detrimental to the health.  I’m just explaining to my dog why her walk will be halved today, it has nothing to do with the lousy weather, my decision is purely based on health.)