Maybe do some exercises to warm up and/or do a posting trot and/or do some no stirrup work or ride a horse that needs leg encouragement lol Idk that's what works for me
I stand on the bottom step of a stair case and push my heels down. Also doiing this exercise while Riseing up and down will help build strength
I use a book called "fit to ride in 9 weeks" and an app on my phone called "30 day fit challenge". On top of it all I ride 4-6 days a week.
Just riding everyday will help strengthen your legs!! Cardio workouts such as running or biking also help! You can also try strength training for your legs, which can include exercises like squats, lunges, and more! One of my personal favourite exercises to help strengthen my body for rising is yoga! It's great for getting back into riding and also helps relieve stress!
You can walk up and down stairs, do wall sits and really focused on your leg calfs than anything. Also, I would recommend trotting withought stirrups when the time comes, to really help :)