The first thing that's apparent is that
the numbers are all in the range 1..6.
the number of letters in the alphabet is comparable to 6 squared,
it's natural to consider
pairs of digits,
the total number of digits is even. It's also a multiple of 3, but it doesn't look as if there's much repetition in the digit-triples, so that isn't a very promising line of attack.
An obvious guess is that
letters are mapped into pairs of digits in some consistent way (there's already a pretty obvious guess, but let's be a little methodical here).
We then notice that
there are quite a lot of 11 and 15, which would work nicely if those were A (first letter) and E (fifth letter).
So the obvious guess is
that at least 11...16 are a..f.
Looking at what that gives us
and considering whether then 21..26 are g..l, etc.,
ba.a.ced which could be "balanced" if 26 is L
we just do the obvious thing, interpreting a digit pair ab as letter 6(a-1)+b,
a_complete_and_balanced_breakfast where each
_ is a (different, as it happens) "out-of-range" digit pair.
Seems like we've cracked it.
(Is there an egg joke somewhere around here?)
In answer to the question of how difficult it is to crack,
I think it took somewhere between three and five minutes to go through the process above, though I confess my very first thought on looking at the string of digits was pretty much the actual answer.