I just did a solo metric with only a cliff bar at ~1/2 and water as the drink. I wasn't too hungry or spent at the end so I imagine there are folks that could do it easily. I imagine if you are already riding a lot guaging the effort wouldn't be too difficult. My ride wasn't super hard in terms of climbing but there was a some wind which was a pain but it was mostly on the way out. Judging what to eat is something that I don't have since my typical ride is 30-40 miles and I don't ever bring anything to eat on those rides. I was pretty much guessing on the metric. If I ever do a solo century I think I would have to guess high though since you never know what you might actually need and when depending on the intensity as mentioned