I'd go with an aluminum frame road bike if your intent is to work up to a 100 mile bike ride. Steel is more durable, but it is also way heavier which means over a longer ride, you are using more energy to keep things moving.
Why a road bike vs a hybrid or a mountain bike? Road bikes, hybrids, and mountain bikes all have different gear ratios, which impact your top speed. There are also weight differences between mountain bike frames, vs road bike frames, vs hybrids that also come into play. The rim/tire diameter impacts how far you travel per revolution. Even the way that tires are designed on these bikes make a difference.
-The gear ratio on a road bike can allow you to hit speeds of 50 mph while you are pedaling downhill. Depending on the mountain bike, your max speed might be closer to 20-30 miles per hour.
-Shocks on a mountain bike or hybrid will slightly rock as you pedal. This is stealing energy from you over long periods of time making your pedaling less efficient.
-Mountain bikes and sometimes hybrids traditionally have smaller rim/tire diameters, meaning you travel less distance per full turn of the wheel.
-In terms of tire design, most road tires are almost bald. This is because on a flat, dry surface you will generate enough friction to keep the tires from slipping. Mountain bikes have lots of knobs made to dig into the road/gravel/dirt, and tend to be a bit heavier. You may feel like you are less likely to slip with them, especially on wet road surface, but over a long ride they will make you less efficient and more tired. With hybrid tires, you're looking at the worst of both worlds. The center of the tire is mostly bald, while the edge of the tire is knobby. If you're trying to go for speed on a hybrid, like a road bike, you will find that the knobs on the edge fuck with the friction you're generating in a turn, and the tires are more likely to skid/slip.
If you're an every day rider going on short 10 mile rides you probably won't feel much of a difference, between a road vs MTB, but on a long ride, I promise you will feel it. I did my first ever century ride on an aluminum mountain bike where I swapped out the MTB tires for baldies to try and make it more like a road bike. My suspension system on the mountain bike could also be locked to make the fork stiff. My first ride took me 10 hours and my average speed was about 16 mph. The second year I switched to an aluminum/carbon fiber road bike and I cut my time in half, avg speed was 25 mph.
Is over training a thing? It sure is. If you aren't using cleats to clip into the pedals to take advantage of your opposite leg pulling up, while your main leg is pushing down, you end up having your quads doing all of the work, and they become overdeveloped. Overdeveloped or imbalanced quadriceps muscles can potentially contribute to a condition known as patella alta, or "high-riding patella". This condition occurs when the kneecap (patella) sits higher than normal in the groove of the thighbone (femur). While often a congenital or developmental condition, it can be influenced by muscle imbalances around the knee. As someone that experienced this first hand, it hurts, a lot. For me, this condition developed over 20 years of constant cycling; (I cycle to commute and exercise.) As long as you can do it safely, without inflaming old injuries, you should find ways to work your other leg muscles to keep things in balance to prevent this.
In terms of training for a century, everyone is different. I spent a year training on a mountain bike and gym bike/spinning before I even attempted a century ride. Before you sign up for a long ride sign up for shorter rides, sag supported rides. Start with 20, then 40, then 60 mile rides. (Sag support means there is a vehicle that follows behind the cyclists to offer support, or pit stops along the route to help with breakdowns, snacks, water ect.) Before I did my first century, on my bastardized mountain bike, I was able to do a 60 mile ride no problem. On the century ride, I hit my wall around 80 miles, the last 20 were pure torture.
Other considerations - learn to fix a flat tire, and make a point to carry spare inner tubes, a pump, and a pressure gauge, or pump with gauge for ANY ride that will take you away from where you can get reliable transportation or a ride to pick you up if you have a flat. In my area, the roads are absolute dogshit and full of major potholes, which are sometimes unavoidable resulting in a pinch flat. A pinch flat, also known as a snakebite, is a type of flat tire on a bicycle caused by the inner tube being pinched between the tire and the rim, typically due to underinflation or a hard impact. This results in two small, parallel holes in the tube, resembling a snakebite. Having your tires at the max allowable psi/barr will go a long way to prevent this, but you will experience more flats on a road vs mountain bike.
I've felt this way for 15 years and I am not trans, although I am part of the LGBT community.