Commuting 36 kms, 5 days a week My workplace is 18 kms ( 11 miles ) away from home. I am planning to commute to and from every workday, 5 days a week.

My stats -

6'1" (185 cms )
82 kgs (180 lb )

I generally average around 22 km/h while going and coming (13 mi/hr). The commute takes about 50 mins, I am in the  zone for 30-35 mins.

I am able to follow this routine, but worry that I am overdoing it. I feel that I am loosing muscle mass.

So far, I do no weight / strength training. Eat a normal and healthy 3 course. 

Can anyone please give me idea about -


Diet regiment. ( I really hope suppliments can be avoided.)
Body weight exercises if any is required. I hope I can avoid the gym.


The aim that I am able to achieve is to keep continue cycling with this routine hopefully generally around 15mph ( 25 kmph )
If you are feeling physically worn out by the commute and you are not used to, well, you are just tiring yourself out and not allowing you muscles to recover between rides. 

Keep riding and your body will eventually adapt. Maybe you want to do this commute every other day for a while, or at a slower pace to help you your body strengthen itself.

Simply riding will build strength and endurance. Doing some body weight core exercises will probably help stabilize you torso and let your legs work efficiently.