In reply to iksander:
I get marzipan, and knead into it a mixture of the following:
dates, dried fruit, seeds, nuts, dark chocolate, hemp protein, any of the random superfood things you get in packets etc...
In general, I take a 500g block on a long day, it fits in a chest pocket so it doesn't freeze and I only tend to eat about a quarter of that... It then freezes well for the next hill day!
For it to work well you have to eat it little and often though, I tend to have a chomp every hour and I avoid sugar rushes/lows that way too.