I had the exact same experience when I was running. It took me ages to build up to a steady 6.0 pace and not feel like I was going to die. In fact, I could cruise at 5.8 but as soon as I went over that, I was done. I knew that my HR for running was about 10 bpm higher than for cycling from a short foray into running around the time my riding started getting more serious. And my HR is generally faster than it should be for my age when I push it, yet, I recover OK. The one thing that cycling helped me with is that I never found it too difficult to run up steep hills. Sure, I'd be going very slowly, but I was running, when others were walking. A speed of 7.0 is *fast* for a beginner and definitely qualifies as a sprint interval. I agree with Oak; the breathing is totally different.