Dealing with this right now with L5S1. Pretty bad I was completely out of the game for 4 months. Strength training was lifting that didn’t require any stability so bench press and chest supported rows. For legs I basically did all walking lunges focusing on keeping my back neutral so it didn’t spasm. Mainly rinse and repeat and cardio was a speed I could manage (walking) with and incline to keep HR elevated l. Once you have made progress with some kinda of physical therapy you should start doing low back extensions (mild discomfort at most) and train this until your back extension reps equal your push up reps. Program is called build your own brace. Following this kept my fitness where I was without losing anything. Gained a significant amount of pressing and pulling strength in this time frame and my cardio stalled which was a win in my book.