My grandpa was a chain smoker since he was a teen and he died at 88 of smoking related issues. His last year's were severe COPD etc but before that he was healthy as a horse. I believe he would have lived until 110 if he never smoked. He was young at heart.

I think anything in the extreme is bad for you. Albeit I'd wish my grandpa was a chronic exerciser than a chronic smoker, either could have killed him at 88.

So I guess it just depends how much you value old age. If you're ready to go and don't value extending your years as far as possible, then do 400 miles a week and ignore the experts and do what makes you happy. If you crave life and dream of blowing out candles on your 100th birthday cake then I wouldn't be biking 400 miles a week.