I am 5 years post tbi and still cannot get my heart rate past mid 90's without bring on symptoms. that being said, I have managed to get pretty active by doing yoga, tai chi, walking and swimming all at a leisurely rate. All of these activities will strengthen your muscles.

I too have a really hard time with the heat and humidity so I take short walks early in the morning and exercise indoors in the summer. On days with heavy fatigue I will shorten the length of my walks to only 10 or 15 min but I will try to do that 2 or 3 times a day. I have never really found a solution to dealing with the humidity other than to limit my exposure to it

I gained 30 lbs after my accident and have managed to lose it all very slowly so it can be done.