I've found that my Garmin 1030 has been a very useful tool ( when it doesn’t have a hissy fit mid ride ) which is actually quite rare now. It will give you a load of interesting stats when you save the ride. One of the really useful stats it gives you, is recommended rest time. It takes a few ride’s worth of data, before it gets accurate, but when it does, it’s actually very useful info. It will help prevent really bad days in the saddle, caused by over training. It sounds like that’s all you’ve encountered, unless there are other issues ( flu / cold / general winter bugs) for example. It’s also worth making absolutely sure you over pack regarding food, if you’re planning an endurance effort, you can always bring excess food back, but if you run out, you can find yourself in trouble. If you want to get some ‘mojo’ back, plan a route, which isn’t one of your usual routes. It should help with motivation to get going, and once you are going, it should start to feel worthwhile again. As for not getting into a pickle in the first place, remember that relative high cadence at a power which predominantly engages your fast twitch muscles, will cause your feeding equilibrium to shift to fat fuelling, which will help to stretch your on board Carb supplies ( in muscles and liver mainly ) further. If you grind at that power ( relatively low cadence and the power that causes your fast twitch muscles to remain dominant ) your feeding equilibrium will be shifted to Carbs, you’ll get rapid carb depletion, and risk the bonk, sooner, if you don’t keep your levels topped up. Grinding ( relatively low cadence ) at suitable power also engages more muscle cells, which increases oxygen demand, and will leave you feeling ‘out of breath’ sooner in any exertion, than if you keep the Cadence higher, at the same power. The actual power ( range ) where this important fast twitch muscle dominance starts to occur, varies depending on concentration of fast twitch muscle fibres, and limb dimensions, but for most normal people, it’s ( roughly ) between 180 and 200 Watts, and the ‘high Cadence’ is roughly 85-90 rpm ( whereas the grinding/ carb hungry mode is thought to be 60-70 rpm at the same power ). The reason that the fast twitch muscle dominance is so important, is that the slow twitch muscles don’t exhibit the same variation in fuelling modes, dependant on switching force and frequency, that the fast twitch muscles do.Power meters are a very useful thing to have as well, it makes it far easier to judge where you are, regarding fuelling. You say you’re an experienced cyclist, who likes hilly rides, so I’m presuming that your VO2 max, Max heart rate and Lactate thresholds, are at a level where you will feel the benefits of getting your pedalling regime ‘polished’. You’ll also find that being mindful of your pedalling dynamics ( how your feet and legs ‘track’ the pedal around the rotation, to eliminate power dead spots at certain positions) will help keep you riding more efficiently, and help prevent getting into difficulties with fuelling / fatigue. TL;DR, have a warm bath, and a long sleep, eat some porridge, then go for a ride somewhere new.