Originally Posted by zebano
I have been commuting to work for a while now. Originally it was on my mtn bike and took about 40 min. A couple months later, I was down to <30 min. I just picked up a used road bike and now it's even shorter. I often hear 30 minutes cited as this magic number where anything shorter than that just isn't cardiovascular exercise. Since my goal is to lose weight, should I find a longer route to/from work that will take a full 30+ minutes?
Also, why 30 min?
Couple of reasons. one comes from the (now rather outdated) idea that if you wanted to lose the most fat, you had to work with your heart rate at 60-75% of it maximum. In that range, you would be primarily using the body's aerobic energy system. People saw that the aerobic energy system utilized relatively the most fatty acids for energy, so they thought, "well, if you want to lose fat, you need to work out in that range". The problem was that at 60-75% MHR your calorie burn rate was so low that you had to work out for at least 30 min, preferably 60 min and up, to burn any appreciable amount of total calories. Remember it all about calories in vs. calories out - not where the calories come from.
Second reason comes from endurance athletes. Endurance athletes need to train their aerobic energy systems for long periods of time because that's where they'll be getting most of their energy from during their events. For them, 30 minutes wouldn't cut it for building up aerobic endurance.
Is your commute 30 min each way or round trip? If it's each way, then you're getting 60 min total on the bike. For your goals, it doesn't make much difference if the cardio comes in one session or two or three. Just add up the time.
Also, do a google search on high-intensity interval training (HIIT) - which a lot of current research is showing to be superior for fat loss and increasing maximum cardiovascular capacity. You may want to add that to your program.