the 2nd workout of the day, some days bad, some days good

It certainly varies (the two lunches), but they are almost always leftovers from dinner the night before that have been split into two even meals.

My snacking between meals is first apple, banana, and then occasionally the treat of a rice crispy square or chocolate. 

Honestly, I fall asleep every night the second I hit the pillow. Probably 11pm or so. Three days a week I am up at 5:15am, and 2 days a week I am up at 5:45am. 

During the week ~ 6 or 6.5 hours/night

Weekend ~8-9 hours/night

Can sleep deprivation cause the symptoms I am having? Keep in mind I am good for the first bit of the run, then hit a really tough spot, then recover from it somehow and keep going for another 30min or more. 

I am usually averaging 7:30/mile (running comfortable), but when that feeling hits me, its like a brick wall.

Do you eat enough and soon enough after your first workout? http://sportsmedicine.about.com/cs/nutrition/a/aa081403.htm

if you feel better after the gel, it might be a nutrition thing leading up to your workout. if you don't already do so, maybe keep a log for a week so you can sit down and analyze what seems to work and what doesn't. definitely work taking some notes on so you can determine the different between the good and bad days.

Sounds like a really good start. Thanks everyone.

#1 - nutrition log of what I eat and when I eat

#2 - Get some protein in my snacks and try to avoid just eating simple carbs

I hope my run tonight goes well.

One more thing: Are you running on swim days or bike days?

I have no problem running on swim days, but bike days are hard. No surprise, in the swim I use mainly upper body, on the bike lower body, and on the run lower body. Consider alternating run and bike days.

If I were having symptoms like these I would not go for another run until I had seen a doctor.

2 weekdays (morning swim, evening run)

1 weekday (morning bike, evening run)

other 2 weekdays (just bike in the morning)

I am leaning towards nutritionist, but I have a feeling that a nutritionist will not be able to help unless I can tell her specifics of my diet (must maintain nutrition log first)

Perhaps by then I will already have an answer.

I guess the Doctor would be worthwhile if there was something more serious going on that I didn't realize. I just recently switched doctors (6months ago) and had a full set of blood work done. Everything checked out okay there. Also my blood pressure was norm and HR nice a low like usual (48 or so BPM resting HR).

Doctor. Sorry, I should have said more. What the OP described (an extremely weak feeling that hits like a brick wall and slows him from a 7:30 pace to a walk) sounds pretty scary to me, especially since it's happening so regularly. Maybe it's just a nutrition issue, but I'd want to get checked out to make sure it isn't something more serious.

Is it something new that started since your last visit to the doctor?

I had certainly happened many times over the past couple years (as far as I can recall), but it is happening more regularly now. Perhaps I am just working out 2x per day more often now. Also, back 2 years ago, I was a gel junkie (I felt like I always needed to have a gel before a workout, 45min into a workout, etc.). So it is likely that may have been a factor in my reduced frequency of episodes (the extra gels compensated for the supposed nutritional deficit I had been running with). 

I almost asked the same question. I've had this happen to me a few times. Light headedness during a run mostly. Really annoying. Maybe dumb question, but how's your overall hydration? 

I would say track your nutrition as others have stated. Also go to bed earlier at night than you are. You should be getting at min 7 hours of sleep but closer to 8 from what I recall.  

I have noticed that my 2nd workouts during the day can at times not be as good and usually I find it because of food eaten and also because of lack of water. 

Also are you trying to do 2 hard workouts in the same day? Anyways  best of luck with things

Yeah, the blood sugar thing looks like the culprit. I am going to experiment this week focusing on more complex carbs for all snacks to avoid spikes in my blood sugar levels (essentially, instead of fruit and any sweets I may normally consume, I am going to have carrots + broccoli + cheese/crackers).

I hope this does the trick. If not, I am still going to be keeping a nutrition log (started yesterday) that I can analyse or take with me to a nutritionist. 

Thankfully I have never been in danger of passing out, and hopefully that will never happen.

Thanks everyone for the information and suggestions. 

I don't think it's a big deal. If you're ramping up volume, your body will respond a bit awry for the first 1-2 weeks of doing doubles while it acclimates to the volume. Happened to me a lot at the beginning of last season, I HTFU'd up, and it went away.

For me, it definitely correlated with how hard workout #1 was. If I hammered #1 and then tried to hammer #2 but wasn't quite in shape for it, I got a big slowdown on #2. It went away as I got more in shape, and then could hammer even 3 SBR workouts in a single day.

I don't think it's a big deal. If you're ramping up volume, your body will respond a bit awry for the first 1-2 weeks of doing doubles while it acclimates to the volume. Happened to me a lot at the beginning of last season, I HTFU'd up, and it went away.

For me, it definitely correlated with how hard workout #1 was. If I hammered #1 and then tried to hammer #2 but wasn't quite in shape for it, I got a big slowdown on #2. It went away as I got more in shape, and then could hammer even 3 SBR workouts in a single day.