Experiment completed. To help inform all of you, I conducted an experiment under strict scientific conditions. The result: eating 3 mince pies for lunch does not make you run faster.
In fact in 30 minutes I only did 3.26K.
I didn't even try to push myself at the 3 minute warning as I knew today wasn't my day, so I decided to carry on plodding. I got back to the start and thought I may well continue for another 250m to reach 4K. I did it except that I had miscalculated and actually did 4.25K. (Typical government accountant). So an underwhelming run (of course we don't use the b. word) became a very satisfactory one. Maybe mince pies make you run longer, not faster. Perhaps I should have taken a smaller dose first.... Or maybe I should let someone else test that hypothesis in future.
Two days off now - maybe my doing 3 runs in 5 days was the reason for my undoing and the mince pies actually helped me.
Yipee kay aye etc, guess what I'm watching now.