To calculate a big number and somehow have it start with 16 zeroes? Yes, I'd say that is pretty difficult.
Was it one zero back when Bitcoin started and the difficulty was at 1? Or one of some other number?
Difficulty 1 is just as arbitrary as any other difficulty. The probability of any individual hash solving a block is (2^32)*(difficulty). On average is requires (2^32)*(difficulty) attempted hashes before one is found which is smaller than the target (and thus valid for solving the block). So at difficulty 1 it took about 4.2 billion hashes on average to solve a block.
2^32 = 32 bits. Expressed as hex one digit = 4 bits. 32/4 = 8. At difficulty 1 a block would need eight zeroes.
While looking at number of leading zeroes can be useful it is important to understand the network doesn't look just at the leading number of zeroes. If it did then difficulty increases (or decreases) would be limited to a factor of 16x (when difficulty changes it could only stay the same, go up 16x, or be reduced to 1/16th) that is beacause each additional leading zero, make the target 16x less likely.
A block is valid if the hash is smaller than the target. Right now that is: 0000000000000000
896C00000000000000000000000000000000000000000000
So a hash with the value 0000000000000000
912300000000000000000000000000000000000000000000 would not be valid despite "having the right number of zeroes".