I just want to mention that I read someplace (yeah I know, unaccredited source from the internet, sheesh!) that interval training is not recommended for most older folks. It was a credible enough source (to me) that I decided to forego it at least until I'm able to discuss with a doc who understands sports medicine as it relates to me. It's something like the guy shoveling snow... Most of our recovery rates have dropped severely, from those times Bitd, and doing true interval training can stress one's heart too much before one realizes. I mean honestly, that sh** was brutal Bitd when I was competing.
I'm not trying to dissuade anyone from getting as fit as they can be, just that the passage of time imposes certain limitations... And a (wo)man has got to know their limitations.
Even though I will never be FAST again, (and personally, I don't find personal bests all that exciting) I still push myself hard and find myself telling my legs to shut up.
Best regards, Eric