Running on empty

Today I went for an early morning 'speedwork' session because I failed to do it last night. I've never done speedwork in the morning before - I usually just do a comfortable 6-10km. I have to run 'on empty' because if I eat something before I go I feel a bit sick. At that early stage of the morning, I failed to realise that doing speedwork on empty would be extremely hard. I was planning on doing 4x800m (in about 3.25) with 20 minutes warm up and cool down and 2 and a half minutes recovery. Running on empty is alright as long as you are going at a comfortable pace, but speedwork requires a significant proportion of energy from anaerobic glycolysis (lactic acid system) - burning carbohydrates. I probably used up most of my glycogen stores in the 20 minute warm up so I didn't have as much energy as I would have liked when I came to do my speedwork. I'd be really interested to measure what fuels and energy systems I used during my run. My heart rate and breathing rate were much higher than usual which indicates possibly using fat as fuel. Anyway, it was a good session, well it felt very much like it was a good, hard session so I am happy.