Firstly very sorry to hear your diagnosis.
My wife was diagnosed with MS in 1989 she had always been a fit active person who ran & cycled. (Rubbish swimmer mind so Tri out of the question).
She only went through a year or so of Relapse Remit MS then it became Secondary Progressive.
Until 1999 you would be hard put to know on first meeting her that she was suffering with mobility issues, 1n 1995 we walked most of the Pembrokeshire costal path in very hot weather.
Since 2008 she has hit a wall with many things shutting down, but she has always battled to keep doing stuff.
I think the thing is to measure your efforts and don’t push yourself if you’re tired as that seems to be when things go wrong.
As cloudnine says PMA helps, secondly email me if you want any more info as after living with & caring for someone with MS for 25 years I would like to think I can help a bit.