Having tried to make it as a horn player I recently sold the damn things and joined a local choir. On the day of a recent choir rehearsal I didn't feel well but decided to go anyway.
All seemed fine until we stood up to start singing. I felt faint and then needed to run out fast to be very sick. Although the culprit was our old friend the Norovirus I realised later that the vomiting was encouraged by the process of breathing in and belting out the sound with the help of the abdomen and diaphragm. After two days of feeling near death I got back to daily walks with my dogs building back up to my regime in the gym designed to build my body strength, posture and balance etc.
At a mere 71 there should be no reason that I can think of to stop you playing the horn. I couldn't see the point in owning two horns which never left their cases so sold them and used the money to upgrade the engine, suspension, brakes etc to make my road going Subaru Impreza ready for use on the race tracks where it belongs.
I recommend driving on a race track as fast as possible as an alternative to playing the horn. You need to concentrate hard, be aware of what is going on around you and then see how fast you are able to corner without hitting the bank. The element of risk is very similar to the risks we take playing the horn and can be scary (another familiar feeling).
I am sure that you will regain the strength to play the horn standing, sitting or even on the march. (See Sarah Willis).
Tony C (74 and counting)