I am the uphill athlete dietitian, in answer to your questions as Pete has highlighted, there is research to suggest that cramping is in fact related to muscle fatigue/neuromuscular over load, more so than an electrolyte imbalance (depending on environment, if it is hot weather again as Pete highlighted then cramping may be caused by dehydration/electrolyte loss).
If it is the first day of a trip, which is the ‘main event’ you have been training for, perhaps pace and effort are greater on this first day due to a number of variables compared to training when back at home, hence resulting in muscle fatigue 2-3hrs in. Perhaps as you settle in the trip, you adapt to the environment, pace/effort settles to that which is comfortable enough to not induce cramping.
Some research based alternate solutions to try for cramping include;
1) Magnesium supplementation (perhaps take this on the morning of the first day of a trip). Be mindful of quantity as magnesium supplement can be poorly tolerated by our guts and can cause stomach upset so test it out before you leave. Aim for approx. 200mg of magnesium supplementation. Also check if your electrolyte table does in fact contain magnesium.
2) In the UK there is a supplement called ‘cramp fix’, but it is in fact pickle juice (and a required taste) again good research has shown that taking this approx. 5-10mins prior to the onset of cramps can help alleviate the severity of cramps or stop them from occurring. Anecdotally I have heard people report that apple cider vinegar has the same effect.
I hope this helps to answer your question.