I used to imagine myself as a steam locomotive and the furnace was being stoked with Mars bars! This helped when energy was flagging a bit.
Another was the ground was on fire/broken glass, so my feet had to have as little contact with the ground as possible. This helped speed and also a lighter step.
Another was imagine string tied to my ankles and over my shoulders; I was holding the string in my hands and as I ran my hands pulled the string lifting my legs high.
Sometimes these things sound silly but the old maxim applies "If it works for you". Actually, I do sometimes use these 'mind tricks' even now.