cellular respiration - What effect does ambient temperature have on the cardiovascular system?

I am sorry if this is not appropriate for this site, but I think it fits so I am asking it here:

I went for a run yesterday, and it was about -8⁰C. I was wearing a couple pairs of shorts, a t-shirt and a long sleeved shirt, which is not that much, so I went with the idea that I would warm up a lot while running. After running for a while, I found that I was tired and out of breath much earlier on then I would normally be. It raised the question:

How does decreasing the ambient temperature affect the cardiovascular system?

There were two things I could think of that might directly affect it. (1) The fact that my body's metabolism has to work to heat itself, and this takes energy. However I am not sure how much of a role this plays since running warms you up anyway. (2) The air being so cold, and entering the lungs, I imagine my body has to work much harder in some way to deal with it. What exactly happens here, and how is the process different from breathing at 25⁰C?

Also, how much faster would we expect a runner to be able to finish a 10km run when it is 25⁰C outside versus -5⁰C outside?