Snow in April!

I seem to have hit a barrier with my running. I am unable to keep up a running pace for as long as I would like, ending up walking to catch my breath. It's not that I am getting physically tired - unlike when I was less fit and would be exhausted after a run, I feel about the same afterwards and would be happy to do other exercise afterwards. I am aware that my walking and running have hugely increased my overall fitness, as well as adding muscle and removing body fat. It's just the actual pacing and being able to carry on running rather than walking that I am struggling with. I'm not really sure what to do but it's very unmotivating. I suppose just carry on and hope that I get over this hump soon! 

P.S. here's a wonderful video I found today (prepare to cry if you watch it!)