Les Mills is the best fitness app by far. The instructors really help to make it feel like you're getting a personal training course.
I'm pretty fit for my age - I regularly run, swim and go walking to try to keep fit and keep the weight down.
After a session of Les Mills if I haven't done it for a while, especially with the exercises that have loads of squats, I can barely move for days afterwards I'm so stiff and achy.