I am a man of 73 and a life-​long asth­matic. After a spell in hos­pital with pneu­mo­nia, I find climb­ing stairs dif­fi­cult because of breath­less­ness and wasted muscles. I also get slight angina if I over-​exert myself. Is it safe to increase my heart rate? I do not want to waste time and effort try­ing to achieve the impossible...