First off a HUGE CONGRATS for turning off the buzz and turning off the fat button, great job, I love hearing stories like yours!
The next thing is STOP comparing yourself to others, this will only frustrate you and maybe even frustrate you so badly you'll throw your bike back in the trash where you found it! Give yourself time to improve, look, not everyone is the same, I use to race CAT 3 level but I wasn't good enough to go to the next level not alone go pro! I seriously believe it's genetic, look when Eddie Merckx was retired from racing for 30 years and had gained a lot of weight was approached by a group of reporters wanting to do a story on his life and what he was doing currently, when they saw how big he had gotten they were stunned and one of the guys asked if he could even ride a bike anymore! LOL!! That sort of [censored] Eddie so since he owns a bike business he got several bikes together and went for a joy ride with the much younger reporters...after 30 years of not riding, and gaining a lot of weight the reporters, who the ones who went were riders themselves, couldn't keep up with him!! This is all due to genetics, if I had done that I would been lucky to go 8 mph for a 1/2 a mile! Some people are more genetically built better than others to do all sorts of different things, some people never even realize or discover they have a genetic gift in a certain area, and some, like your friend, maybe slightly better than you genetically OR he has a lot more training time then you do and he could actually be genetically inferior to you but you haven't come to that level yet to find out.
I know people who have been riding or running for years and are just plain slow, and I've known others who just started doing this stuff and are faster out the gate, again it's genetics. I'm kind of in the middle of the road I think, not superior and not inferior, just average rider who with training was able to push myself to the cat 3 level when I raced. That racing was 30 years ago so I'm older and now I live in climate less favorable to riding all year round as well, usually by about mid fall my average speed is up to around 20 to 21 mph, after a winter hiatus and spring starts my average speed falls to about 13 to 14 mph and I have to start all over again; this year I'm hoping to correct some of that hiatus stuff by going to a gym.
Yes a road bike will be more stable for that aero position you want, and you'll probably want aero bars to maximize the benefit.
The gear part is more tricky because we don't know what kind of roads your ride, is it mostly mountainous or mostly flat, or somewhere in the middle? Do you have any knee issues or ankle issues while pedaling? If so you want to pedal at a slightly higher cadence in an easier gear, if your cadence is currently around 60 you want to work that up to around 75 to 80, that doesn't mean you need to go faster, it means you need to drop your gears down so that you spin slightly faster but you're still going the same speed you were before at slower cadences. Understand that? Mashing gears at slower cadences will hurt joints, so ease up if that's the case. And when you climb hills keep that spinning ratio up high and use your gears to accomplish that.
So in that light I think you should go with starting out with compact gears, this is where the chainrings are 50 and 34 teeth, that ratio will make it a tad easier than standard which are 53 and 39. The larger that outside ring gear is the faster you can go but you also need to strong enough to push those gears and with time you will be but for right now the compact gearing is your best option. Also the lower number is the inside gear, the 34 tooth gear is easier to push up hills then the 39 would be. There are other gears too like 46 and 36 which is more of a cross bike gearing and 48 32 which is now becoming popular on gravel bikes.
Now let's look at the cassette, the less teeth it has the harder but faster you can go and vice a versa for the more teeth. For your particular skill level I would be looking at a ratio of 11-28 range which is typical I believe on most bikes.
To show you an example of how these gear ratios work here's a handy dandy gear calculator that can vividly show you what a change in gears and cadence can do: http://www.bikecalc.com/gear_speed So if I enter your tire size as 700x25c with a cadence of 70, and a chainring of min of 34 and a max of 50, and a cog (cassette) of a min of 11 and a max of 28, then hit calculate and follow the graph you see that if you were climbing a steep grade for example and were in the 34 front chainring and a 28 rear gear you would be going at about 6.7 mph at 70 cadence, and if you change to the direct opposite gears your top speed would be 25 at 70 cadence (assuming of course you were fit enough to go that fast). Now if you change the gears to say a standard chainring of 53 and 39 and leave the cog alone you'll notice the speed jumps up about 1 mph more, that's because that range is more difficult to pedal in and is made more for the advance rider which means it would be more difficult for a beginner to use. I hope that made sense. As you get better and you find you are maxing out your top end for the gears you have, which I doubt that will happen because averaging 25 mph for 20 miles is somewhat of feat, but if you do then you simply change your chainring gears. Personally I think you should have put on any bike you buy a rear cog with 12-32 ratio with compact chainrings, plug that into the calculator and you'll see why, this will make it easier to pedal in while your strength improves. Like I said gears are relatively cheap to replace, and the best time to do that is when you think you have outgrown your gears and you need to replace your worn out ones anyways.
