I agree with what the others have already said.

The right gear to be in is the one that is not too hard and not too easy, and this changes depending on what you are doing and what the environment is doing around you. The shifter on your left controls the front derailleur - and thus moves the chain from the big ring to the small ring where your pedals are attached to the bike. The small ring is for when you want to go easy, the big ring is for when you want to push hard. The right hand shifter therefore controls the rear derailleur, and moves the chain up and down all the cogs attached to your rear wheel. Moving it to a smaller ring makes it harder to pedal, and moving it to a bigger ring makes it easier to pedal. It just takes practice to get the hand motions down and knowing where you are comfortable. My advice is don't look at what others are doing, because for them it'll be completely different. As an example, this morning on my ride we were hammering down into a draw with a bunch of rollers. I prefer to move into a harder gear (we also had a tail wind) and use my muscles more - so I went to the big ring up front and small ring in the back. My partner prefers to "spin" - so he went to the small ring up front and a big ring in the back. We both accomplished the same thing - zipping up and down a bunch of tiny hills at 28 mph - but our gear ratios were totally dependent on our personal preferences for this kind of riding.

I also recommend that you ride frequently with ever increasing distances, but allow yourself enough time to rest. I did 55 miles this morning and I'm beat. I ride 30 most days now and we kept a pretty steady pace of 15-17 the whole time, which is what I usually ride at. There was also low winds with occasional gusts (meaning easy riding). But those extra 25 miles? Oof. At 15 mph, it's almost another 2 hours in the saddle. So ride ride ride ride ride, but don't forget to rest too. That's the only way to get your butt callus and make it long distances.

I also recommend clipless shoes. I've never ridden in anything else. The best part is, when you are tired, you can employ different muscles to move the pedal (ie, pulling "up" on the pedal instead of pushing "down" as we normally do) and it'll help you go farther.

And as far as speed goes - it's too variable to say an average. Most long touring rides I've been on typically keep a 15mph-ish pace. But in a large group, you'll have the people in the back and the people in the front who will come back for those at the end of the line. That's what I did this morning - kept circling back for our slower members. As long as it's a no-drop ride, you won't get left behind (hopefully).

HTH.