As a starter for ten:
Sugar is actually pretty darn good. It diffuses well, can be broken down immediately by cells needing a boost, and it's pretty simple to get hold of.
If your hypothetical creatures have 'sprint cells' (like the exact opposite of fat cells) lining their cardiovascular system that contain high concentrations of a glucose syrup/oxygen/adrenaline mix (built up over time), then when they're in need of a boost a special hormone/chemical can be released into the bloodstream. This chemical would cause the walls of the sprint cells to break down, releasing huge amounts of sugar (which cells need for high performance), oxygen (which cells need for high performance) and adrenaline (or something similar).
The advantages: Instant high energy, a feeling of heightened alertness, crazy strength (adrenal surges can do really weird things), and a general feeling of invulnerability. This also scales at the same speed as the cardiovascular system of your creature and can be 'tuned' for varying energy release patterns based on hormone strength/number of sprint cells etc.
Potential side effects include: Epic crash after the buzz wears off, Liver failure, Kidney failure, Irreparable brain damage, Cardiovascular disease, Increased risk of stroke, Hyperglycaemic shock, muscle strain and diabetes.
Use with caution.