I own a Giant Toughroad SLR 2 bike, it's an hybrid bike and I'm planning to register myself in a Seattle to Portland ride.
It's a 2 day - 200 miles ride from Seattle to Portland.
The question is would this bike make it or do I need a road bike? What modifications would you recommend to use my bike?
Disclaimer, I've never done a long distance ride.
First, don't worry about the bike - worry about you. If you need a different bicycle, you'll figure that out pretty fast while you're training to do your 200-mile ride.
What's the farthest you've ridden on that bike? 30 miles? Even 50 or 60 miles is only just getting to the edges of what riding 100 miles is like. Fatigue is cumulative - never mind just running out of intramuscular glycogen - all the microtrauma your muscles endure riding a mile is piled on top of the microtrauma endured in the previous miles.
Ever done 80 miles or close to it? Because you're going to need to work up to at least that just to be able to do 100 miles in one day without getting surprised by something like debilitating cramps at 94 miles.
Are you guaranteed to have something like that happen to you if you don't work up to the distance? No. But if that does happen to you on the first day of a two-day, 200-mile ride, what then? And issues like that are probably about ten times more likely on the second day.
To be able to do 200 miles in two days, an 80-mile ride in one day is going to have to be fairly easy.
When you first start riding distances like that, the final miles are hard. You won't just be tired, your leg muscles will feel like they're just about to turn themselves inside out, and they oftentimes will - grabbing at you with cramps that can literally make you fall over off your bike. Your arms and shoulders and neck can be awfully painful. Your fingers numb. Your back muscles spasming. You might get saddle sores, or worst-case, bad blisters on your feet.
Even if you work up to that distance slowly, there's simply nothing else that can prepare your muscles for the 6, 7, or 8 hours of continuous effort needed to ride 100 miles or so. Again - because fatigue is cumulative.
Coming back the next day and doing another 100 miles is going to be even harder. Your muscles simply will not recover in that short of a time.
So how do riders do it?
By working up to it. Over time.
So you need to get riding. Work your distance up over time - you have several months.
Go out this upcoming weekend. Plan on riding a bit further than your longest-ever ride - you should probably aim for at least 30 miles. Squeeze in 2-3 90-min or longer rides during the week. Allow for some off-days to recover. Add 5-10 miles per week to your long weekend ride.
After about a month of that - say you're doing 50-60 miles on Saturday - come back on Sunday with a 25- or 30-mile ride, really easy. Start adding miles to that, too.
Plot out your planned distances. I'd say you should probably aim for 70/70 back-to-back rides about a week or two before the ride.
Because it's that back-to-back that's going to be the toughest thing to handle.
And don't ride at all for three or four days prior to the 200-mile ride. By that point, all you can do is fatigue yourself.
And while you're working your distance up, there's a chance you'll discover all kinds of issues - your shoes don't work because when you get to 70 miles you start getting hot spots on your feet. (You'll know why they're called "hot spots" if you get 'em...) Your saddle might not work. Your hands might start going numb after four hours. Are those guaranteed to happen? Again, no. But they're all possibilities.
And note that I didn't mention your bike not being able to handle the distance at all. It won't have a problem at all as long as you do routine maintenance.
If that sounds hard, it is. But it's better than that 200-mile ride turning into a misadventure you'd rather forget.
How far have you ridden in a day? The bike will be fine, as the comments say the question is whether you will be. A road bike is more efficient and will get you there easier, but there are many people who could do the ride on a hybrid or worse. Are you one of them? Decades ago when I was in college I rode a century on a basic bike with a Sturmey-Archer 3 speed hub shift.
Are you viewing it as a race or as a ride? Do you want to finish or do you want to beat a lot of people? Many of the people will have better bikes, so lesser cyclists will beat you. Is that a problem for you?
If you are energetic and can enjoy a 30 mile ride on the bike you have you can do this. Can you do a 30 mile ride, stop for lunch, and do another? That is most of the way to 100. Some training to increase your endurance will make it much easier. Some people find the range of hand positions on a road bike helps endurance considerably. Others seem not to care.
I'm a roadie and can't imagine doing this on anything but a proper road bike, but we had a number of people show up for our shop ride on hybrids and knock off 30 miles before lunch with no problem. I'm sure most of them could have done centuries easily.
As far as modifications, the first thing I would suggest is slick tires. They lose less energy flexing when they hit the road. A narrower, higher pressure tire is also good. Change now, not in a couple months, to make sure it works for you. You should also make sure you have low enough gears for the climbs involved, but likely you do. If you do decide to use another bike, make sure to get plenty of experience with it before the race. You want to make sure you are happy with the fit, and that takes time.
Have fun!
As others already said/commented, the real question is if you would make it. The bike will do, almost for sure.
The bike needs to comfortably carry you and some spares for you and for itself:
spare tire;
inner tube(s);
repair kit (the tools needed to remove wheel&brakes, plus patches to fix inner tube);
pannier rack and pannier(s) or at least a large saddle bag (are you thinking about a backpack for some of the above mentioned stuff? think twice ... or at least try once!);
saddle + handlebar grips: you will never figure out if they are comfortable until you spend something like 4 hours for 2 consecutive days on them;
narrower tire, more efficient, but generally there is a trade off between comfort and efficiency = if you go faster, you will spend less time on the saddle, so you may accept a rougher ride
Again, my recommendation would be to ride at least 30 miles for 2 consecutive days. Then you will have a better feeling about what you need. Although the positioning of saddle/handlebar/pedals are highly subjective, if you experience feet/hands/groin going numb, the way to remediate to these are easily found with google and some adjustments.
The bike should be fine. A road bike would be faster but you can do a few things to make your current bike better suited for long distance road biking:
Another problem that haven't been raised by anyone (as far I could see):
You won't be able to avoid the heat - and in July it could be sunny and 30+ Celsius.
I once did a 180km ride (120 miles) in a day, starting at 5:30 or so. The most horrible part was the last 50 km (30 miles) at 30+ degree Celsius (some 90 Fahrenheit) with the summer sun. That kind of heat can be soul-destroying, and you must prepare for it.
You might enjoy heat, and you might have the luxury of taking a three hours break in the mid day and pedal late into the night and very early in the morning - but prepare for heat.
(all my so-called training happened either early in the morning or late in the evening, at 25 or less Celsius and no sun).
