You managed the walk in the end. The fact that you felt better after the chocolate could indicate that your blood sugar was low, but after only two mile it's most likely that your fitness level is too low for much hill walking.Two hours before a walk you must eat good carbohydrate food, that will give you an energy reserve for a few miles, top that up on the move with energy bars and the like, a banana has good potasium content, cashew nuts are high in calories and don't firget to stay hydrated, drink 500 mls energy drink before starting off, then drink to thirst while walking. Hills are always going to be harder than on the flat, the only way for them to get easier is to walk them regularly, it takes time, a lot of time. The 3 Peaks ascents are quite short by comparrison to those in the Lakes or Scotland, but they are sharp and with the distance in between you have to be fit to complete the route in a reasonable time. The ascent of Ingleborough and the descent of Whernside are particularly steep.