I ride 40km on an unpowered bike for the first time in about 30 years the day before yesterday and surprised to have no real pain.
Keep it up. The hills are terrible at first, but if you stick with it, they become easier and easier. I've just been out for a 40 miler, which finishes with a 5 mile climb that gets steeper and steeper. I can remember struggling up that hill on my first electric bike, but today I was chatting to a young guy on an MTB, who does that climb every day on his way back from work. I was able to talk normally all the way up without even puffing. The more you stick at it, the easier it gets, even for a 65 year old.
One thing that does help is to lower the gearing a bit. I always had a 39T inner ring, but I changed the rear cassette from 25T to 32T bottom gear. That made a massive difference on the steep hills.