Hi @Sophilina welcome to the club no-one wants to join. I would suggest you do the exercises they tell you to do. I didn't always do them after my mastectomy and I have a little bit to go for strength and stretch.
Fatigue after chemo and radiation slowed me down a lot but now 12 months on I am gardening again. I can't do it for as long as I used to but for me the letrolze medication I now take daily for 10 years means I can't stay down on my knees for a long time but age and weight add to this.
I am sure you will be able to continue a career in horticulture.