// The string we are working with.
string str = @"1010101010010100010101101";
// The number of bits in a 64 bit integer!
int size = 64;
// Pad the end of the string with zeros so the length of the string is divisible by 64.
str += new string('0', str.Length % size);
// Convert each 64 character segment into a 64 bit integer.
long[] binary = new long[str.Length / size]
    .Select((x, idx) => Convert.ToInt64(str.Substring(idx * size, size), 2)).ToArray();
// Copy the result to a byte array.
byte[] bytes = new byte[binary.Length * sizeof(long)];
Buffer.BlockCopy(binary, 0, bytes, 0, bytes.Length);
// Write the result to file.
File.WriteAllBytes("MyFile.bin", bytes);