Running, Cycling & Interval Training?
 

So this isn't strictly a tri question, but it seems like y'all may have the most relevant experience, and I may do a duathlon. So....

I did a little bit of running 5 years ago, and worked my way up to a half marathon. I'm getting back into running, and am also using an HRM. I noticed that my runs are at a very high heart rate -- zones 4 and 5 for the whole run. And that's at my normal pace, which I tend to think of as a bit slow (10 minute mile). It's much easier to control HR zones on the bike.

So, I'm wondering a few things about how to do interval training for running, and mixing that with cycling.

My concern is that running more than a few times a week would basically blow the curve, since I'd be spending too many days working too hard.

In theory, I could run a few days, and cycle at less intensity on the other days. However, I'm concerned this won't mean enough running time.

Any suggestions?