I've heard of that book on the mtb forums. I think Ned Overend shows that you can still be fast over 50, but he may be an anomaly.

I used to do HIIT with hill repeats on the bike and running, even bleachers but I start to get knee pain on my left side, so I limit the intervals just so I 'save' my knee for the years to come.

Looking forward to your entry into the Health Challenge. The channel appears to be a great asset for those 'healthy' steemians.