Unless you live in Kansas, you should be able to find a ride with some elevation changes.

If you're not training to race, just do natural intervals based on the road, hammer up a hill as fast as you can, then rest, or spin up the hill then hammer down in your highest gear.

It really comes down to this: To ride faster, ride faster.

I don't do anything based on a stopwatch, but I have a 4 mile route around my neighborhood with about 500ft of elevation gain. I will ride 4 laps of that 1 day a week hitting the uphill parts as hard as I can. Another day is a 24 mile out and back that is hilly with 2 hard climbs and another day is just 30 miles of gentle hills that I ride at a moderate pace.

But ultimately, you want to be able to extend the length of time you can go at higher effort levels.

As you progress, you will be able to spin at the same cadence in a higher gear, but it takes a while, expect 6 months to a year of work to see serious improvement.