What happens when running is the only exercise you do for two months?

livemint.com:

Where did the muscles go?
For starters, if you follow, and trust, a structured training programme, you become a faster, more efficient runner. Your stamina improves. You can run your benchmark distances in less time and also run for longer durations of time.

Cardio exercises such as running have a positive impact on your heart health and you will find that your resting heart rate starts coming down gradually. It means that your heart has to work a lot less to do its job of pumping blood to all parts of your body. By the end of the first week, I already felt better than I did in the first long session, with which I’d started the programme.

By the third week, I was able to push through despite the heat. By the sixth, I didn’t care about the heat, humidity or hangovers anymore, I would finish my run no matter what. The running prowess I made was empirically tested on race day and I managed to come very close to my fastest ever 10km timing.

I had no doubt that structured training and running regularly would make me a better runner. That’s exactly what happened. But what about the rest? Since I stopped working out, I started losing muscle mass.

So much so that when I returned to the gym, my trainer asked me how I’d burnt all my muscle. I lost a significant amount of strength, which I found out when I tried lifting weights. I couldn’t squat, bench press, military press, clean or deadlift weights that I used to without breaking a sweat.

While my stamina had improved making body weight exercises easier, lifting workouts were a struggle. I also struggled hitting the pull-up reps I was used to. All this, including muscle mass, can come back quickly with proper training.