I was interested to hear a couple of years ago that the official snack of the British mountainbike team is Jacob's Fig Rolls, and not just because Jacob's is a sponsor. It's a good mix of food groups for a slow release as required, much better than more obvious sugars and starches. The reason I was so keen to discover official sanction was that I had worked that much out for myself, and always carried a jumbo pack of fig newtons on the bike, usually Jacob's brand.
Like Anto, I'm keen on raisins. My people own vineyards, so I'm a connoisseur (!) of raisins!
When I was a rugby player in training, we were given steak and potatoes swimming in butter, no greens, and the guy who created this menu was reputed to be one of the great sports trainers of the world... Just throwing that in for interest, as it is a method that might stage a comeback, and I don't pretend to know any better.