<human>: Condense the text into a few key points

Hi Reddit! (First post so please tell me if i do anything horribly wrong.) I noticed a phenomena that I presumed is relevant only to new runners. This might be a completely usual thing and, if so, I apologize for taking up your time! I'd be interested to hear your opinion.

I'm a new runner; I'd say I've been properly interested for about 3 months. I find that on my runs I'm gasping for air within the first couple of kilometers. However as I struggle on, maintaining my pace, I seem to need to breath less and less. Eventually by the 12km mark I'm breathing intermittently with my mouth shut.  Around the 15km mark I seem to hit a wall energy wise and start failing to maintain a pace as everything gets 'clunky' and forced. However even then my breathing is very easy.

I'm asking you guys if this is a normal phenomena for new runners? Most reports I've come across suggest that most new runners are limited by their muscle soreness much more than a shortness of breath? If it is interesting why do think its occurring?

Anyways some relevant facts that might help your ponderings: 
1) I'm a mid-20s male ~90kgs
2) I'm NOT asthmatic 
2) Over 5km I'd struggle to maintain a 4:30/km pace
3) For the 15ish km runs I run about 6:00/km pace

PLEASE NOTE: I'm in no way concerned about this. I'm sure as my body adapts to my activity everything will normalize. Just thought it could be interesting to ponder about. :)
<bot>: I'm a new runner who struggles to get enough air despite having plenty of 'go' in my legs. Thought is was an interesting scenario to discuss!