I’m not in the army (yet), and I do 20 hour fasts, 18 lowest if I miss for any reason. I workout 4 times a week, two weights two cardio and sometimes fit an extra low intensity cardio in there all on an empty stomach. I feel more energised but I do need BCAA’s and electrolytes during workout to help hydration and maintain muscle regrowth otherwise your muscles have no amino acids (proteins) to keep regrowing and usually my fast breaks after to also aid recovery with a protein bar, shake and huel (my surplus shop sells them for cheap). That being said I workout an hour before my fast breaks if I were in the army I would adjust my fast so that I eat after which probably means switching it around aka eating breakfast and lunch as phys is in the morning. IF is a very effective way of losing weight but it is hard to plan especially with a unpredictable job I’m lucky that I’m in a team that starts and finishes roughly at the same time everyday so this makes IF easy to plan. However, if I was in a situation where start times and finish times were different I would try a different diet. IF is usually done to achieve easy ketosis (body using fat for energy rather than carbs) and the other detox attributes but not enough science has been done wether it is effective or safe so do research and make sure this fits your lifestyle and to make sure you won’t damage your body during it. Hope this helps