I'm a 46 year old male. I mountain bike pretty hard at least one day a week. I've been doing High Intensity Interval Training off and on for the last year and it's really improved my Vo2Max (which is my #1 constraint when mountain biking).
Up til now I've done about 10m/day about 5 or 6 days a week plus one hard 1.5 to 2h ride with folks much faster than me.
I'm considering changing things up, maybe resting every other day but doubling the amount of HIIT I'm doing.