Speaking of protein and snack, it is hard to beat greek yogurt. You can also add granola to greek yogurt if you are not to keen on the taste.
edit: It would be interesting if anyone has juiced heavily with beet juice prior to racing or similar. Beets are supposed to be extremely high nitrate concentrations and can have positive effects. Andrew Jones’s group at the University of Exeter has studies on the endurance-boosting effects of beet juice, which they previously reported can extend time-to-exhaustion by about 15%.
here is an abstract, I can dig up the whole journal later.
http://www.ncbi.nlm.nih.gov/pubmed/21471821