OK, so the running and HIIT aspect.
Easy part first: running. For the next 3 weeks, slow down your pace to say 8:30 to 9:00 min mile so that you can hold a conversation and run for a longer period of time - say 35 to 45 minutes.
Why? Because
a) your body starts burning fat after about 20 minutes of aerobic exercise - the longer you run, the more fat you burn but this effect is only good up to 60 minutes based on the research I've read. Thereafter, aerobic exercise tends to break your body down
b) fat needs oxygen to be metabolised, so easy breathing is better to access fat but make sure your heart rate is still reasonably high - about 70% of max if you know what that is. I use a heart rate monitor.
Now the hard one: HIIT. Why am I saying this is hard? If done right, HIIT is anaerobic which means without oxygen. It is characterised by short burst of high intensity work which cannot be fuelled by fat metabolism - ie ketones - bc we've established that fat metabolism needs oxygen.
Instead, in an anaerobic context your muscles will use glycogen which is stored in a part of your muscle cells. Glycogen (of which we store about 1500 calories in muscles) is effectively glucose. So what happens is you are using up glycogen stores when you do HIIT.
Why is this a problem? It's not. What is a problem is how to replenish those glycogen stores so you can do HIIT again soon. Your body would do this naturally via fat metabolism but the process is slow. Athletes doing anaerobic sports need carbs to replenish glycogen stores quickly. And so here's the problem: we need to keep carbs away from you for the next 3 weeks.
So you have a number of choices with respect to HIIT:
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continue as you are doing and see how it goes. If you are sluggish it will likely be bc of glycogen depletion; let me know and we can assess what to do. If you only do twice a week you may not notice a difference.
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To avoid being confronted wit this problem, stop HIIT for 3 weeks and do something else. Heavy weights are great btw and should be part of everyone's routine esp as you get older.
Personally I would choose 1) and reassess if you get sluggish ... there are a few options we can try.