Hi there,
I'm a member of staff on an Air Cadet squadron, and I've experience working with the Army cadets too. All cadet forces provide a well rounded mix of activities - fitness is not a necessity, and you definitely do not have to do lots of physical training. We have plenty of fit and unfit cadets, and all find an activity that they can excel at. If you are unfit and want to become fit, we can help! But you are not required to do anything you aren't capable of. We want you to challenge yourself, but we want to you have a good time while you're doing it - if we adopted the typical 'basic training' style of screaming, push-ups and lots of running we'd have very few cadets!
Just to give you a flavor of some of the activities we do (Note: this will vary from squadron to squadron based on staffing and equipment):
-Flying and Gliding (we are the 'Air' cadets after all)
-First Aid
-Shooting
-Sports
-Navigation
-Duke of Edinburgh Award
-Fieldcraft (military skills, camping etc)
-Drill & Parades
-Volunteering
-Leadership and Teamwork skills
-BTEC qualifications
-Camps in the UK and Overseas
In the Army cadets, you'll do similar activities, but there's more of a focus on the military skills and shooting, and you will have less opportunities to fly (obviously).
In the Sea cadets, swap the aviation activities with boats and sailing activities.
Hope this helps inform your decision. Joining the cadets was the best decision I ever made, i've developed so much as a person and had fantastic opportunities I never would have had. Feel free to ask any questions.