I just got a new bike (more on that later), and I've taken two rides on it so far with a friend. I commute by bike to work, and I've raced another bike I recently sold in a crit series. The two rides I've done the last couple days were around 60 miles, and I know that sounds like a lot, but I'm currently running four miles at the moment, so 60 miles really is within my fitness level.

About 10 miles into the ride my back started to hurt both rides. It always hurts when I'm working harder to sustain my speed. I think my bike fits fiine. I'm 5'9 and ride a 55cm, and have long arms. I know I can't rule out the fit, but I think it's more because I haven't ridden in a while. My back doesn't bother me much when I'm riding at slower speeds of 14mph or so.

Does anyone else who sits through a long snowy winter have these problems early in the season?

I have the same problem, but no real answers either. I can tell you that as I've been increasing my mileage over the spring, it's gotten better. My lower back used to hurt after about 20 miles, now I've got it to where it doesn't hurt until about mile 40.

When my lower back hurt on my bike it was because I was too stretched out, once I got a shorter stem my back pain instantly went away.

Ive dealt with the same issues. For me strengthening my core helped. Also if you're new to riding you may just need to get used to the position. Running 4 miles is very different from riding for 20 miles. Even if you think your fit is good you;d be surprised what a pro fit can do.

I'm the opposite. I found slightly less back pain when I stretch out and flatten my back. I had my stem lowered to get a more flatten position. When I scrunch I hurt more. Also think pushing higher gears adds to more back pain. I think it's the side muscles, not sure the technical name, that hurt and get stiff.

My lower back hurts like a @%&^$#^ from mile 5 to any mile, beggining to end of the season, high or low gears.. Sometimes off the bike, in the office, it will kind of lock on me too.. Im 27. many injuries including bilateral shoulder dislocation.. back to the bike pain, I figure if I go really fast and get my adrenaline way up there, I don't feel the pain as much.

Can you provide more specifics? How did you go about strengthening your core? I know I can google core exercises, but I'm curious what worked for you.

+1
Listen to bikerjp's advice. This is why many suffer needlessly from back pain. They do the opposite of what it takes to eliminate it...having poor posture on the bike. I also believe core strength is over played as a solution.
OP, you need to 'rotate your pelvis' forward. Think pressing your belly button to the top tube. If this puts too much pressure on your balls then change saddles and find one with a cutout. If you don't release your pelvis and flatten your back you are in for a long tough road. Riding hunchback, scrunched and upright on a road bike is not only slow but it pile drives your spine and sets you up for arm, neck, upper back and trap pain.
Your back hurts because you are over extending it by planting your sitbones rearward instead of rotating your pelvis forward. Many believe that the root of back pain is lack of flexibility but position trumps flexibility when it comes to pain.
Road bike position is really the opposite of what many believe and why so many struggle with it.
Hope that helps....the anatomy of what I describe is depicted below:

work on our core strength, same used to happen to me.

"Core strengthening" is such a buzz word it's kind of sickening. I'm sure there's something to it, but the way the term is thrown around it reeks to me of one of those concepts that will be debunked and laughed at in future years (eg. "spot reducing"). Again, I'm sure that's not the case, but people use the term too generically and vaguely, which is why you get questions like these, "what does core strengthening actually mean?"

Right on the money. One can have very 'average' core strength and have correct posture and ride strongly without pain.

This is true. For whatever reason, after I started a regimen of planks, side planks and supermans my pain/soreness has been eliminated. Everyone is different. With this in mind I would get professionally fitted first, if you havent done so already. Make sure the fitter is aware of where you are experiencing pain.

I don't understand why the image says you need to twist your seat to get comfortable. What's up with that? Seems very counterintuitive.

I've had the opposite experience. Maybe it is my saddle, or all the saddles I've tried, but my lower back and body in general is more comfortable when I don't twist my pelvis forward. For what its worth the Fizik Spine concept seems to show that as well. They show more flexible riders as having their pelvis not rotated forward, less flexible riders have their pelvis forward. I don't know what the answer is but I'm not sure there is one answer that works for everyone.

https://www.fizik.it/spineconcept/#/how-it-works.html

I don't get that either.

I have my saddle every so slightly twisted to the right and it is comfortable, fwiw

Generally speaking people are asymmetrical. Trying to get an asymmetrical body on a perfectly symmetrical bike doesn't make much sense.

I want to reiterate that I only feel pain when I'm moving at a strong clip. if I'm relaxed and just spinning at 14-15mph I feel no pain...

+1. The challenge is to figure out in what direction. I ride with my saddle straight ahead but have experimented with Cobb's suggestion to rotate it. Some good if not top riders do rotate their saddle a bit. If getting low is the objective which it is on a road bike then pushing the saddle off to the side a bit may afford necessary clearance. I think a cutout saddle accomplishes the same end.
Cheers.

But having good posture aka a flat back isn't about 'good' or superior flexibility is the point often lost. An inflexible person can have good posture on a road bike. I am not particularly flexible. The point is, a more aggressive torso angle can be achieved by rotating the pelvis forward and it takes extraordinary flexibility to achieve an aggressive torso angle with pelvis rotated back because you are asking your vertebrae to do the impossible....high relative angulation. This position causes strain and injury. I have said before that the biggest reason that riders don't rotate their pelvis forward is because of weight on their balls. The key is to find a saddle with perineal relief to allow rotating the pelvis forward so you can ride with a straight back and aggressive torso angle to get low out of the wind and put strong force to the pedals. Good posture whether walking, typing at a computer or riding a bike is a recipe for the best back health.

So what kind of saddle do I need?