After reading Bro Haidada's comment about exercise to keep fit and keep the manhood strong and hard, any suggestion on what kind exercise that you can recommanded.
I for myself would do running and situp to strenghten the lower back and the lung. So you won't be out of breath after few minutes of humping and your back will not be sore just after single round. Cardio will make you look younger. I also do weight lifting to boost up my upper body and legs. It make you so sexy after you take off the clothes and showing some chest and bisec muscle to the girls. They will think you are superman It works for me all the time.
Suggestion????
How about some sperm retention exercise too???

(1) Balanced diet but have high protein food like egg yolk, steak, oyster.  But this is dangerous and you need to check very careful your cholesterol level.

(2) Constant excercise your heart like cycling or jogging

(3) More stretching instead of weight lifting

(4) regular life like wake up early and sleep early

(5) less alcohol & less smoke...

There are three main types of exercise we must do:

1. Cardio - it strengthens your heart and lung. It is important as you said that you wont be out of breath.
2. Weight training - to strenghen your muscles, you must work out your chest as it is the so called Shirt Muscle. MM likes that also. Strong legs are also important. Sit up is good as you said to ensure you can continue the in and out motions.
3. Stretching - is also important. You can do this while watching TV.

The main thing is to do it regularly. I work out almost 5 days a week when i am in Hong Kong, not travelling i mean, already for many many years.