Well, we really don't know because OP stated that they enjoy long runs. I'd think they are doing some long runs in this case...
But, implicitly, I was trying to make the point that 2 minutes is nothing unusual. The HM course could have been downhill, OP had a tail-wind, etc. The OP's 5k PR could have been in XC, so their 5k PR on the road or track would (very likely) be faster.
To me, what the OP's HM and 5k results say are that they have some okay aerobic conditioning, but lack the conditioning for faster running... like the 5k or a fast HM. I think he/she just needs more (smart) training and more consistency. Like you said,
I think the OP just hasn't done that much work yet, and it's premature to conclude from the 1:18 HM that the OP should throw themselves into the HM and marathon distances.