Audax rides

Eat, eat, and, just when you can't face anymore, eat. Drink a bit when you eat too.

That's good advice: most people who struggle haven't eaten early enough. Eat in the first hour and you will be fine (as long as you eat in the other hours too!)

For most people, a 300 is not such a step up from a 200 in that it is just a full day ride. From a 300 to 400, is usually a bigger step - although smaller in % terms - as it goes into the night.