Yeah, I don't think your age is holding back your potential on running stamina, but it may be a challenge increasing factor. I have noticed that for me aging does effect many things related to working out, like recovery time, how fast performance goes down, if having a break from training, due to injury or for what ever reason etc. And the same things done the same way may not produce the same results, as it did in the past. So you may just have to switch something up and approach the running training a little different.
Also one thing to consider is, how has your body composition and weight changed since your best running days? I was at my best cardio performance level, when I was younger and played basketball on a pretty high level, and it would be tough to get back to that level, cause' I'm 20-25 lbs heavier now. I believe I could get to the same cardio performace level, if I really wanted, but not with this much lifting in the pictrue. It would have to be the main goal. Remember that increase in muscle mass increases oxygen expenditure. So if you have build more muscle, then that alone can affect your cardio.
Anyway my point is try to approach it differently, if the same things don't work so well anymore. Usually it's stuff like taking more time and smaller increments in training to get closer to your goal. If you're not doing lower intensity and longer lasting cardio already, then I suggest adding that in. Doing both low and high intensity cardio training has allways benefitted the performance in both areas for me.
For the HIIT running you could try something like 30 sec sprint - 60 sec rest, do that for 10 mins and add one sprint per HIIT session untill you're at 15 min. Then reduce the rest period to 45 sec and do the 10 min again and go towards the 15 min. Then reduce the rest to 30 sec and start from 10 min again and go towards the 15 min. If this still works, then do the same thing from 15 min to 20 min. Stuff like that for example with small increments and easier WO's along the way.
I'm not discounting the hydration or other possible factors and those are definitely things to look into. And if your overall recovering ability has dimished and you're still pushing as hard, then it's worth to note, that not fully recovered and tense muscles tend to recuire more oxygen/energy. I notice this when doing cardio the next day after a heavy squat or DL, for example. Or when just being overall beat up from working out. Foam rolling, mobility excercises, streching may help.
