I have walked up 2 hills, both of them when my wife and I were doing loaded touring on our tandem. One was steep gravel and the rear wheel just spun though we didn't fall. The other was 19% and we just blew up. The interesting thing to me in both cases was how hard it was to move the bike uphill, actually much harder than riding it had been, but because we were going so slow walking, our HRs were lower. We noticed other riders, not touring, who were walking their bikes up hills we had ridden up. This was in our mid-60s and neither of us is a strong rider. We just ride a lot. We did a pass climb between the Czech Republic and Germany, Pass Strasse, 15% IIRC. We just stopped every few hundred yards until our HRs became slightly reasonable, then had at it again. That said, it's way easier to start on a hill with a tandem than a single.
It also helps a lot to have good pedaling technique. An excellent training aid is to practice climbing some moderate hill in a huge gear, so that you are pedaling very slowly, say 50 cadence, and not moving your upper body or prying on the bars, just your feet going around. When I was training on a SS, I'd climb really steep hills at a 35 cadence, seated. Of course that means foot retention. No foot retention and you're walking.
Anyway, as I'm sure other have already said, gear your bike so you don't have to walk. On our tandem at 150 y.o., we have a 26 X 40 low gear. On my single, I have a 26 X 30 low gear. These are totally reasonable gears. I have a 24T granny ring for the tandem, but don't need to put it on . . .yet. The trick for gearing is to use a small chainring so that your cassette won't have huge gaps. IIRC, Machka once posted about riding loaded up a mountain pass in Colorado at 1.5 mph. That's probably a lower limit! It's still faster and easier than walking the bike. Bicycle calculators are very useful for figuring out what gear combo will enable you to climb That Hill seated.
__________________
Results matter