Hello Nikos, nice to hear you are looking to be active!
I'm 20 years old and ive been diagnosed with MG for about 4 years now. I exersise very often and have a gym membership, have done for the past few years. Exercise like alot of people have said in these posts, is good for everyone but we do need to be careful with ourselves. How much you do depends on how you feel during the particular day, if you feel really weak and drained it wouldnt be such a good idea to engage in physical excersise as it would do more harm than good. On the days you do feel like you have energy, try to go on the treadmill or engage in some exercise, no matter how little. You dont really need to start on 20 mins if it feels too much for you, how much you do depends on how you feel. Throughout time try to make tiny piecemeal improvements to your excersise like walking for an extra 30 seconds or something that challenges you, but of course without question safe for you and youre able to handle. Before excersise i suggest you also eat ALOT, juice vegetables and make smoothies, vitamins, superfoods or anything to give you that "boost" in preperation before you excersise. Remember to fuel your body and have your husband with you at all times to ensure your safety especially if youre feeling to push yourself. All the best!