shoulder pain

I'm assuming you're talking about muscle pain. The key to tracking down the problem is assuming the position that leads to the pain and seeing precisely what it is you're doing that places stress on the muscles that end up hurting. It may be a low-level of stress that causes problems because of prolonged time period, i.e. your longer rides.

Almost certainly in your case it's because you're supporting your weight too much with your arms. For sport riding, you should try to put almost no weight on the bars. Support yourself with your lower body by clamping your legs tight in against the body of the bike and using your back along with your lower body to keep yourself upright instead of getting lazy and doing the natural thing, which is to lean on your arms. It may help to keep your elbows always bent and your arms feeling loose, never tense.

The added benefit is that the bike also handles better because you're not inadvertently adding steering inputs every time the front wheel hits a bump, which is something you can't avoid doing if you're leaning on the bars. Same goes double for braking.

Of course using your lower body may mean you end up with sore muscles there, at least until you get used to it. Mine were really sore after a day at a track-day school where the instructors reminded me to do this, which forced me to keep at it. But it will improve your riding technique and at least your shoulders will get a break.

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