If you had a cadence counter, it would be a lot easier.

What are you spinning comfortably NOW?

Raise it 5 to 10 RPM.

When that gets comfortable, raise it again. When you can't raise it anymore, give it awhile, and raise it again.

You will be at 80-90, in no time. Then you will wonder how you ever didn't do it, because it is sooooooooo much easier.