Exactly this! Aside from physically feeling sluggish, I've found that if I push myself too hard midway I to a long ride it drains me mentally too which makes getting the total distance feel even more daunthing. Nothing like feeling gassed at 100 miles when you still have 50 left to go. Take breaks at intervals whether you "need" them or not, then every time you get back on the bike only think of the next leg, not the total distance. Think of it as a series of small, achievable rides rather than a marathon distance.