Cardio for a Mtn Bike Marathon

Question
Hi, I am planning on riding in a 50 mile mountain bike race on July 4 in Breckenridge, CO.  There is over 2 miles in elevation gain and most of the race is above 10,500 feet.  I live in CO so altitude isn't much of a problem and have been training since August of last year.  This winter I have been riding a stationary bike at home and lifting weights for upper body strength.  My legs are strong.  However, my cardio vascular is a problem.  When I ride at home I do 25 minutes at a level 13 cross train and average 140 heart rate and complete about 7 miles.  I have been building levels slowly and my legs are doing great.  However, when I get on the mountain bike, which is rare in the winter, my heart and lungs struggle on extended climbs.  I try to run periodically knowing that it's good for cardio vascular but I have a bad knee so I can't do as much as I'd like.  Do you have any recommendations that will help me?  Thanks in advance.

Answer
Hi Kevin,

Have you tried any breathing exercises to strengthen and expand the lungs?

Since your problem seems to be more related to breathing and getting enough air in the lungs,

Here is a site that has articles with exercises to do and explains in more detail why it is needed.
http://www.livestrong.com/article/22608-breathing-exercises-increase-lung-capaci...

The increase in lung capacity should also help the heart work less.

I don't know if you smoke but if so STOP, and do anything you can to protect the lungs and heart.
Hope this helps

Have a great day and check my blog out sometime at www.thebicylenut.com or my new radio show at www.blogtalkradio.com/the-bicycle-nut

Let me know if you have any further questions

Jerry