I rode my first Imperial Century yesterday (actually a few km more due to a navigation error!). Here are my thoughts on the experience and what I think I've learned.

Here's the ride:
https://ridewithgps.com/routes/7943489
https://www.strava.com/activities/1107865521

Total moving time 6h40 - avg. 25.3kph, 164km

1) It was a lot harder than 100km! I had ridden 100km two weeks earlier as a training ride, which left me with plenty of "gas in the tank". The century was a somewhat hillier than this, and I ended up riding half of it alone, so there was a mental challenge of just pushing myself forward without a group to set pace with.

2) Study the route. Once I ended up alone, navigation became much more important, and the design of the route meant that it doubled back on itself and crossed over in a figure-8 at one point. My simple Garmin with its breadcrumb trails shows both routes in these cases, and it's very easy to pick the wrong one if you're tired. I did this and added a few km to my route having to backtrack. The next ride, I'll set up important cue points on a laminated card, so that I don't miss a critical turn.

3) Start much slower than you think you can go. I wanted to ride with a group and latched on to some riders who seemed to be going at a pace I could maintain. I was indeed able to keep up with them for the first 75km, but I exerted too much effort in doing so. I should have kept an "easy pace" for the first half, rather than a "moderately fast" one. The second half of the ride was much tougher as a result. Unfortunately, I didn't look at my average speed for the first half, but for the whole ride it was about 25kph, so I imagine the first part was 28-30kph - too fast to maintain for long distance with climbs at my current level.

4) Traffic makes it harder and much less fun. The route went through a mixture of suburban areas, large highways, semi-rural countryside and national parks. I was surprised at the amount of traffic in the rural areas, which were very often large trailer-trucks which are quite unnerving when they go past you on narrow roads. The traffic density meant riding single file and keeping to the side of the road where the road surface is much worse. For a future ride I would avoid such routes and find something far less busy.

5) Road surface condition makes a big difference. Continuing from the previous point, having to ride on poor surfaces (and this was often the main road surface, not just the edges) really slowed things down and made it uncomfortable. I was riding a carbon CX bike with 28mm tires, but wished I had fatter tires at several points. The sections with smooth road were fine on the 28s, but the poor quality road surfaces made it a lot harder to keep pace and the vibration and road buzz was tiring.

6) I more of less got my nutrition sorted, but made a possible mistake of eating too much during the half way break. I ate a bacon & egg sandwich, which sure tasted good at the time, but was a bit heavy in my stomach once I set off again. In total I ate:
Breakfast 30g Whey protein drink with 300ml milk.
3 x oatmeal cookies (about 200 kcal)

"Brunch": coffee & bacon+egg sandwich

On bike:
3 x Clif "Bloks" cubes - half a packet (not 3 packets!),
about a palm-full of dried fruit/nut mix (100g?)
Half a Clif Bar

c. 2 litres water with electrolyte tablets

I've read of people suggesting eating the equivalent of a Clif Bar (c 250kcal) per hour, but there is no way I could fit that in. After 100km, it didn't appeal at all. I took a gel, but didn't feel like eating it. Not sure about the total calories (I couldn't measure the sandwich), but it was probably about 800-1000 kcal. Too little? Maybe, but I wasn't particularly hungry at the end of the ride.

7) Company & Scenery. Because the ride was catering for 3 different distances, there was only one other person riding the same distance as me. He arrived at the halfway point in a different group and left before me, so I did the last 90km riding alone. I found this more of a challenge than riding with company, partially because I couldn't draft a larger group as save energy, but also due to the added difficultly of setting your own pace and motivating yourself to keep it. It just felt harder, but to be honest, I probably would have found it hard in any case due to overexerting myself on the first half. One downside of riding in a big group (particularly one going quite fast) is that there no breaks or time to actually look at the scenery. We didn't stop at all (apart from a couple of traffic lights) for 75km. I would have liked to have stopped and looked at the scenery because we rode through some very pretty areas, but I was "head-down" focussing on the road and riders in front and hardly saw a thing - which is a pity, I think!

So, to conclude, I think I've found out a few things about what I like and don't like on a ride:

I don't like to ride so hard that the experience becomes a painful survival trial.

I prefer to ride in relaxed company (even if not actually talking...)

I want the opportunity to take in the scenery, or even stop and take photos.

I don't like roads with lots of vehicular traffic

Having a smooth road or a bike that can handle the rough stuff comfortably makes it more pleasant.

I want cycling to be fun, not a exercise in masochism.