First of all congrats on the ride! Back to back 90 mile days is impressive regardless of how long you've been riding.

As for gearing: First any advice you get from me can be contradicted by others and both points of view can be correct.

If you're sitting in your "biggest" gear (big ring in front/smallest cog in back) you do need to be using your other gears. Generally, you should keep your cranks spinning at 90-100 rpm or so (some will say higher). If the road is flat and you are strong you will probably spend most of the ride using the front big ring but you should use your rear cogs to find gear combos that you can easily spin and stay with your group. You don't need to try to count your rpms or go out and buy a computer with cadence (though some swear by that). When you have an easy spin going you can "feel" it.

So get out of that small cog and use the rest of your gears and look for that sweet spot where you are spinning at the speed you want to be going and not overly straining either the leg muscles or the lungs.

If you're doing 180 mile weekends this will come to you easily.