Importance of long run

What kind of running were you doing over the past 7 weeks?  How fast did you try to run the 6?

If you just raced this weekend, it could simply be fatigue from that race.  That would be my best bet unless you had really cut down the mileage over the past couple months.

How much are you actually averaging (ie, not 'probably')?  1.5-2.5 mi runs aren't very much.  And unless you are really doing a lot of them each week, you may not even be running 12-15mi/wk.  Which is not a lot of running any way you slice it.  You're going to lose fitness by running less, not necessarily by just doing a shorter 'long run' by itself.  Anyway, on that kind of weekly mileage and doing a race this past weekend, I don't think it should be totally surprising that you might struggle a bit with 6 today.

I don't think those 1.5 to 2.5 mile "hard" runs are worth a crap, unless they are done as a tempo run once a week, or every two weeks, with a mile or so on either end as a warm up or cool down.  Interval work, which will also be longer than 1.5 to 2.5 miles for the total workout, is what will build speed for you.  The long run also has a place as a "stress" workout. 

Bottom line, you're not running much.  You can get faster on your mileage, for a sprint race, but you need someone who really knows what they are doing to structure it for you, and you'd need to be doing A LOT of swimming/biking on top of it.

I'll throw this in for consideration: the weather is warmer now than it was 7 or so weeks ago (although I can't speak for Nashville's weather lately). Outdoors runs are going to feel harder anyway, perhaps compounding the problem. Not so much from a leg tiredness but an overall perspective.

You are not running much (your volume has been cut more than 'slightly' from what you have indicated--30-40% by my guess) and you pushed the pace on a run right after doing a race.  It was not an 'off day'.  It's what you should expect under those circumstances.

I know there are those who don't like to hear stories of what my kid is doing because it "doesn't apply" or whatever......but it fits here as well as it does in many instances.

He runs fast sprints......easily sub 1 hour for 750M/20K/5K.  His 5K time off the bike when he is trained up is in the 16:30 range.  He can't do that right now.  The reason he can't is that he is coming off a track season where he was concentrating on the 800.  He went 1:54.2 in the 800 and was happy with his performance and training....he rarely ran over 10 miles in a week and the triathlons he did during that time were based on base miles that he had put in prior to track season.(his fitness is obviouisly HIGH)  His 5K time right now is about a minute off what it should be, and a 5 mile run for him is no picnic.  He'll get back to form fast enough, but my point is that what you are specifically doing in training matters greatly.  You can't look at one run where you felt like crap and wonder what is going on when you've not been doing much running......no matter how fast you are, or how fit you are.

When tri season is over he'll concentrate on XC season and get his 5K time down to 15:XX or under......but he won't be able to touch a 1:54.XX 800.....yet.  Next year he'll very likely go well under that.....but it's because of a LOOOONG body of work, with specific training objectives along the way. One bad long run among very few long runs......meh.

Earlier you said you did the sprint "this past weekend".  I assumed that meant this weekend, having been the one that just passed.  If it was a week ago, then race fatigue should not have been a significant factor.  In that case you are down to just looking at how much you are running and how hard.  You're not running enough to make any run of distance very 'easy'.  And when you decide to do one 'hard', then you are more likely to find yourself struggling to some degree.  Sounds like that's what happened.

That's fine.

Swim more.

It all depends on your goals, etc.  But, generally, 3-4 is "fine" as long as 2-3 of them aren't just 1.5-2mi brick runs.  At your paces, those aren't even 15min of running.  It's hard to prepare yourself "adequately" (again, depends upon goals, etc.) for a sprint tri with a 3-4mi run at the end in that fashion.  Most of your run workouts should probably have a minimum of 3-4mi of total running, including the warm-up and cool-downs.  There are many ways to structure your actual training and the composition of your running (in conjunction with the other two sports, of course) that should be determined by your constraints, current abilities, and goals.

Weight matters, but for a couple pounds you have much more to gain by learning how to understand the big picture better than having another detail that is more likely to distract you from your goals.  At least at this stage.

How many mpw were you running when you could do your 6:50 pace half marathon last year?  To me, it simply sounds like you have been running a lot less since last year and you're trying to run at paces that you could when you had higher run fitness.  I don't think run fitness will degrade a lot over the course of 7 weeks...but it sounds like you're comparing yourself to 12 months ago...after admitting that you also took it easy in the fall.