Someone else has kindly worked out the maths to save me the problem, and suggested some default settings for drag coefficient etc. (http://www.gribble.org/cycling/power_v_speed.html).
I used that to determine that the power requirement for us doing ~25 km/hr will be around 115 W, giving a total energy requirement for the 20 km race of ~330 kJ. Accelerating up to 25 km/hr 15 times will need another ~49 kJ, giving a total energy requirement of ~380 kJ. This does not sound much (90 kcal for those who like old units).
But, that is the work done by the rider (er...me!), who is not that efficient. Stanford University says ~20% (http://large.stanford.edu/courses/2011/ph240/maher1/), which means that to deliver 380 kJ, I will have to consume 1900 kJ, which is ~450 kcal.
To calibrate this, 1900 kJ is nearly 2 Mars Bars (1095 kJ each), so I will need to eat quite a lot to maintain energy balance.
The calculations are available here.
The things left to do are:
- Do this calculation for the swim and the run.
- Estimate how much stored energy I have before I hit 'the wall', so I know how much to eat during the race - I know that after about 2 hours of exercise without eating (running or cycling) I run out of oomph, so this will probably do for a calibration.