Watch your cadence. Pushing big gears is murder for your lower back. Spin, spin spin. Get off your bike every so often & stretch your back.

When you get back on the bike, you need to start slowly. Do 10 miles at an easy, spinning pace. Rest one day. Do another 10 miles. Assess how you feel. There's no point in hurting yourself badly & being out of commission for a week.

Re: upper back & neck pain. I get that on longer rides too. Try painting the alphabet with your nose while you're riding. It gets your neck moving in lots of different ways.

Here is a nice stretch, if your back will allow it.

Sit on the floor & put the soles of your feet together. Your legs will be forming a diamond shape. Let your knees drift down towards the floor, but don't push them. This will open up your hips. Next, slowly lean forward. You'll feel this wonderful stretch in your back. If you curl forward (as if leaning over a beach ball) you'll get a nice stretch in your upper back. Let your neck hang.

I'm not a doctor & know nothing about disks, so if your doc told you not to do this, don't! I just know it feels wonderful...