There are many reasons
– You might not have eaten enough of the right foods in the prior day
– You might have not let yourself fully recovered. Running again while still not recovered from a previous run and not fully refuelled, and/or still sore from a previous run, can start to “burn up” your muscle instead of “burn up” fat. You have to monitor this type of progress.
– If you have lost a lot of body fat and are lean now, you may have run out of body fat to use as energy during the run. You may need to readjust your diet if you’ve finished losing weight, and go to a “maintenace” diet of some sort instead of a “weight loss” diet.
– You might not have replenished your glycogen stores. That means you need a quick injection of sugar (optimally, dextrose, but even a Gatorade type drink will do) right after your run. Generally avoid sugar, but right after the workout is when you need to refuel your glycogen stores quickly to avoid the muscle-wasting process.
– Your sleeping patterns may have changed or you got jet lag. This can disrupt a training program or stall progress. Recover by sleeping a little extra.
– Eat more on the days that you run, than on the days you do not run.
– Most diets have too much fat, but there is such a thing as too little fat. However, sometimes some diets have way too little fat or needs readjustment (depending on what your goals are, loss, gain, maintenance). If you’re bulky, then this is probably not it, but if you are skinny then this point becomes very important.
– You might not have been eating immediately after your last run. Your muscles needs to be refueled quickly after a run, so that means eat something very healthy soon after a run. Perhaps protien shake immediately followed by a healthy meal within an hour. Something like that. (Contents of the meal depends on the person and goals but put a lot of emphasis on protien and slow-release carb)
Also, do not run when you don’t think your body has fully recovered. If you try to run while your body has not recovered, then you can end up wasting muscle or going into such a very tired state like you are.
If you are also, simultaneously, trying to do a bulking program (muscle building) then in certain circumstances, aerobic exercise is sometimes mutually exclusive from anerobic exercise (weightlifting), especially for hardgainers (people who don’t easily gain – aerobics can burn muscle rather than just fat – especially if nutrition is not proper). So it can amplify your tiredness if you try to do two programs simultaneously. I’m assuming you aren’t – but it’s nice to remember this little point;
Anyway, this is all stuff I’ve learned recently, some of them from here, and some of them from books I’ve been reading…
There is no doubt other factors and other advice you should hear. Keep your body refueled if you don’t want to feel tired! Also, not all the advice here will apply to everyone. But it helps to have a good understanding.
At the very least….. definitely don’t skimp on the nutrition that comes right before a run and right after a run.