Final Bike Practice - What to eat?

We went down to do the Triathlon cycle course this morning,
because the weather forecast for tomorrow is awful, and Benjamin does not like the rain.
There was a northerly breeze, which seemed to get stronger on each lap, which means the return leg of each lap was a serious struggle.
We did the 22 km in just over 60 minutes - average 22 km/hr.   Not quite the 25 km/hr I would like to do, but it's ok - I keep reminding myself that the target of doing this with Benjamin is to complete it, not to be competitive, but I keep getting 'go faster' thoughts.

I did an experiment and did not eat during the ride - just a negligible
energy drink to stop me turning into a prune.  I think I was starting to run out of oompf on the last return leg, but it was not bad - I suspect that not eating will encourage the muscles to store more glycogen, but I haven't checked if this is true or not.   Will definitely need to eat for the race.

Ridewithgps.com says I used 430 calories (I am sure it means kcal, not cal), but it doesn't know I was pedalling the equivalent of a 70 kg bike into the wind, so it should underestimate.
I got a cheap heart rate monitor from tesco a couple of weeks ago.  It said my average heart rate was about 155 bpm, and says I used 891 kcal.   I guess heart rate should be a reasonable approximation to power output - I must read up how this works.

So now I must do some sums and decide what I need to eat to keep going at full power  891 kcal is 3.7 MJ, or over 3 Mars bars.   I'll have to try to calculate how much energy  I use in the swim.   Sounds like a job for a rainy afternoon tomorrow!