Hyperion (Saturn's moon)
Your brother is
Lapetus (another moon of Saturn)
I run around my friends,
Hyperion is orbiting around Saturn and there are other moons and particles inside his orbit.
I am not that wide, but very long,
Hyperion is an irregular shape moon.
Some are behind me, but all are within my brother.
There are other moons and particles that are behind Hyperion's orbit, but Lapetus is by far the most distant of Saturn's large moons (so others are inside its orbit).
Some of the young ones run with me, but they don't really count. It's cold out here, but even worse for my brother.
It's cold in space. It's worse for Lapetus because he is deeper into space, on a bigger distance from Saturn.
I look grey and dull, but I can go quite fast, even though I'm dusty and full of ice.
Hyperion is composed largely of water ice with a small amount of rock