Kevin, thanks for the OP. I actually was planning on writing up a very similar OP and posting it for tips and feedback on this topic. For the past few years I have been working on getting/staying in better/good shape and putting on more miles. When I transitioned to UL a few years ago I would put in around 10-15km a day, as when I was a traditional or lightweight backpacker, that was my limit. I soon grew restless, but did notice and enjoy the added energy I would have after hiking for swimming, cooking, foraging for wild edibles, fishing, etc.
Last year with a 3.8kg (8.4lb) base weight I did a 5 day section hike and put in at least 20km a day, sometimes more--one day (and this is my current personal record) I hiked 36km (22.3 miles). I felt good doing it too, though I was quite sore and stiff in the morning at times. Over the past year I regularly have hiked 20-30km on my section hikes, and feel like in another year or two that my new "normal" days will be pushed up to 25-35km. On my last section hike a week ago I did 41km in a day and a half, and felt that if I would have had more days at my disposal, I could have kept up a similar pace.
Some things that have helped me:
Snacks, and snack often. I will eat a Snickers or granola bar 3-4 times a day in between meals. But as others have noted, don't just rely on carbs and sugar rushes. I eat a lot of nuts, beans, and olive oil on the trail in addition to pasta, rice, and oats.
Simplify food routines. Saves time and energy, and gets food into your system faster and easier. Most of my breakfasts are non-cook (Flapjack oat bar, dried fruit, and nuts are a combo I have been really into for the past year or so), and a lot of my lunches are non-cook or easy cook. I save the more complex meals for dinner when I can sit and relax and take my time to focus on cooking. I will often eat a late dinner, around 7-9pm, and before then have a big snack around my "at home" dinner time around 5-6pm to tie me over.
Figure out your body's optimum hike time. This is different for everyone, and a lot of variables are involved. I am a night owl, not an early bird, for example--on and off the trail. After I break camp and hit the trail, I am just not ready to rock and roll. I take it easy, hike a slow to moderate pace. After lunch my energy levels are up more, and I pick up the pace. My big energy bump comes between 4-8pm or so, and that's when I nearly always feel the best and can really tear up some trail if I want to.
Don't push too hard. I take a lot of breaks, stop and take a lot of pictures, jump into lakes on a whim, and stop and chat to any friendly people I meet. The more fun I have (and this goes for a lot of things in life), the more efficient and driven I am. I have canceled trips before if things go sour and suddenly my inner voice says, "Okay, this sucks." So what if an overnighter becomes a day trip? Last spring, for example, I hiked a new trail I didn't know much about. It was fine at first, but then at the end of the day I found myself hiking through residential suburbs and then on roads going by farms. Not wanting to pitch my tarp next to a big farming plain, I just hiked to a bus stop and went home. If I was on a section hike (or eventually when I go on a thru-hike), I would have called it a zero day and spent the night in the nearest town to hit the reset button and start again the next day.
As soon as you feel a warm spot on your foot, stop and take action before blisters set in. Loosen laces, change socks, put a band-aid on baby blisters before they turn into monsters, etc.
Not sure what my next goals are. I am very happy with my roughly 20-30km limit, and think that 25-35km might be as far as I am willing to push. I recently read a blog of a triple crown hiker that claimed (and I believe them, there was lots of documentation) that their daily average on the PCT was around 50km. I admit that I am impressed, but I can't say that I have any interest in getting to that level. It did humble me though, as that is just about double of what my hikes are. I was happy to know there is someone out there that is on that level. I can't help but inwardly giggle when I talk to people and they are surprised to know how far I can hike, when there are others that can blow me away.