Training for long distance riding

Hi, I’ve recently fell in love with cycling and I bought a $150 cross (city?) bike and put about 700km on it in the last 2 months.

The longest ride I’ve done so far is about 170km in one day, which was part of a 280km trip that I did with a one day break in between two days of cycling. I’ve done other rides like 180km in two days, etc.

I would love to do more long rides, but as of now I’ve suffered back and butt pain (😭) and my speed is pretty terrible. I was mostly cycling in Japan’s street roads without cycling lanes so I was slowed down by cars and traffic lights, but Strava says I’m averaging around 17km/hr.

If anyone could give me training advice to improve my speed and endurance, I’d really appreciate the advice.

Looking forward to any recommendations!

(By the way I just recently bought a Brompton c line for various reasons and will probably be using that primarily)