I am in a very similar situation as you. I will flat out admit that this being the interwebz has a huge amount of variation and personal experiences that are given as if they are fact. That being said, I still come here and read a lot in order to try and see what is working / not working for others. Please note that I say I 'try and see' because after being here long enough I notice that some people exaggerate or leave out some important information. Others just want to pound their chests and / or build post counts. As you are already seeing, it is important to listen to who you want to and disregard the rest for a little while. If nothing changes, try changing who you are listening to and again look back after a few months and adjust again as necessary.
FWIW: I'm a 47YO guy, 5'11", and a runner. I do ride and paddle on occasion. I've signed up for my first triathlon (sprint, so I get a taste of it) and so I'll be including swims after these next 2 Half Marathons I've signed up for. The reason I mention all of that is: I've recently lost a lot of weight. Honestly, about 15lbs so far this year. I'm down to around 167 lbs (I'd suggest adding 2 lbs to that because of my scale) at the moment. For me, I bumped up my Miles Per Week and I have been really watching what I'm eating. I'm not on a diet, but I certainly have been watching my diet.
The one thing I would suggest you try is to severely cut your caloric intake one day a week. I read about intermittent fasting and I realized that I most certainly have / had a 'stretched' stomach. In hindsight I think I had forgotten what it was to be hungry. Either I was eating out of habit or eating when the first pangs of hunger started. And, my portions were too large. So, I started cutting way back on the one day a week I didn't exercise. Hungry Mondays were born. I don't do a 500 calorie fast, but I likely eat between 700 - 900 calories. What I will say is that when I only eat a very restricted diet one day a week, it really makes me examine my portions for EVERY meal. As an aside, I eat vegetarian 2 days a week which makes me examine what I'm eating every meal. So, being much more focused on what and how much, perhaps with the added benefit of a slightly smaller stomach size and now realizing what hunger really feels like has all added up to me losing about 17lbs this year. Having said that, I still would like to lose some belly fat. I'm expecting that as this continues, I should get a flatter stomach. Every time I hear my stomach growling I think of it as shrinking in size. It helps to refocus my mind on meeting my goals rather than focusing on the short term hunger.
So, my questions to you are:
1) Have you read the sticky at the top of the Health and Fitness forum? If you haven't read it, read it. If you have read it, re-read it.
2) How do you estimate your calorie intake? If it isn't reading labels, measuring portions, and writing things down, then I'd start doing that.
3) How do you measure your heart rate? I have read that keeping your heart in your target zone can make a big difference in whether you are going to lose weight, gain cardiovascular fitness, build endurance, etc... I wear a BT HRM and check my rates after most every run. For me, it's a solid way to learn just how I really am doing during my runs.
4) What exactly is your goal? Your post mentions that you'd like a tighter stomach as well. So, if you want to focus on that, you certainly can but I think that some of the other things you are doing will have to take a back seat. I don't think that doing ab exclusive workouts is going to make the large difference, but they will give you something to show once your body fat percentage is reduced.
My opinion is that you have plateaued. You've been doing this regime for a while and your body has finally adapted to it. You'll need to make some changes to confuse your body again and allow you to drop the fat percentage / lose more weight. Personally, I'd pick a goal at this point. I'd likely stay focused on your hobbies and the triathlon for the Summer and then focus on the stomach for the next Winter. I'm not saying you can't do everything, but I don't think I'd have much success with all of that (and everything else I have on my plate).