I've never been very comfortable running. I have always gotten shin splints in the past and just stopped.

Now, although still heavier than I'd like, I'm in the best shape fitness wise probably in my whole life. I started cycling this summer and my aerobic fitness has improved 10 fold. I've been swimming too and my lung capacity is so much better.

I'm training lightly for triathlons next year with the goal of a half ironman in 2013. I've been running intervals of 1 minute running and 2 minutes walking once a week for a few months. This past week I did a full ironman challenge at my gym and ran/walked 26.2 miles over 7 days.

I noticed a couple of things that have me somewhat concerned. I feel like I'm in good enough shape now that I should be able to run farther/ longer between walks. Yesterday I walked a mile at 3.5 mph then ran 1 minute at 7.0 mph alternating 2 min walk at 3.5. In one minute running my heart rate got up above 180 and it was almost like I could feel the blood draining out of my body. Once I started walking I could recover rather quickly.

Why is my heartrate going so high when I run? It doesn't get near that high when I bike or swim unless I'm sprinting all out.

I feel like I can't go any longer than two minutes running or I'll risk passing out. I feel very weak after 2 minutes. I feel like it should be improving, but it's not.

If I run slower (5.5-6.5 mph) my knees and shins hurt. 6.5-7.0 allows me to get a full swinging stride with less pounding and I haven't had a much trouble with my shins hurting this go around.

What can I do to improve? I want to be able to run longer, but I just don't know what to think about this heart rate/weakness feeling situation.

Any thoughts?