As others stated: Binary Search is the key to speed! I assume you know about binary search algorithms. I think, you need not fill the keys to get a fixed record length.

As you already have a sorted file, simply seek to the files center by dividing its size in bytes by 2. Then read one line. This one is to be discarded as it is most surely incomplete. Get the current fileposition as your corrected value for the file's center and read the next line in order to find out whther you hit the correct key or whether you have to advance to the upper half or go back into the lower half.

again you will always have to read one line and discard it.

You might miss one line using this approach:

1230 value AAA
1235 value AA
1240 value A
[download]

imagine, your lower bound is 1230 and your upper bound was 1240. Getting the center of those 2 might land at "235 value A". This line will be discarded. advancing one line will hit your upper bound.

in such a case you will have to read all the lines from lower to upper bound to check for a hit.

This is just a rough idea i've never tested!