Just my .02 cents - I basically ran intervals all summer long (last year) and it backfired on me, big time. Yeah, I leaned out but lost a ton of muscle mass. Not to mention, feeling faint, dizzy and chronically fatigued. It took me almost a full year to correct the "problem". If I were to do it over again, I'd do intervals once a week and weight train the rest of the time.
40-ish and feeling fine.