Stick with 5 mile rides. Give your butt and the rest of your body time to adjust to riding.

I started with 5 mile rides. Go easy for 500 miles.

Then step it up to 10 miles rides.

Keep going up in miles when you feel you can.

Learn to snack, drink, and stand up to pedal in a high gear range while you are moving.

40 miles is where many have problems as your energy level begins to drops.

Keep it Fun and take rest day as you need them.

The racers that I have rode with always spin. They only use the big ring when actual racing.