Start Exercising Again When Your Body Won't Do What It Used To

I want to start exercising again, but my body isn't what it used to be and I'm not sure where to begin. Here's my situation:

My age: [e.g. 45, mid-50s, early 60s]

What's holding me back physically: [e.g. dodgy knees, lower back pain, a shoulder injury from years ago, I get out of breath going up stairs, I'm carrying extra weight and worried about my joints, general stiffness — or just "everything aches"]

How long since I exercised regularly: [e.g. haven't done anything in 5 years, used to be fit but stopped after the kids, haven't really exercised since school]

What I used to enjoy (if anything): [e.g. swimming, walking, football, cycling, gym classes, dancing, running — or "nothing, I've always hated exercise"]

What I have access to: [e.g. a local park, a gym membership I never use, just my living room, a swimming pool nearby, a bike in the shed]

How much time I can realistically give it: [e.g. 20 minutes three times a week, half an hour most mornings, only weekends]

What I'm hoping to achieve: [e.g. just feel less stiff and tired, lose some weight, be able to play with my grandkids without getting knackered, get off blood pressure medication, feel more like myself again]

Please give me:
1. A realistic, gentle starter plan for my first 4 weeks — broken down week by week, with specific exercises or activities I can actually do given my limitations. Start easy and build up gradually.
2. Safe alternatives for anything that might aggravate my specific issues — e.g. if I can't do squats, what can I do instead? If running is out, what's a good swap?
3. A simple 5-minute warm-up and cool-down routine I can do every time, designed for stiff or older bodies
4. Honest advice on when to push through mild discomfort versus when to stop — I don't want to make anything worse
5. Three signs that it's working, even before I see any changes in the mirror — so I don't give up after week two
6. One thing I can do today, right now, that takes less than 10 minutes — just to get started

Keep the tone encouraging but realistic — not preachy or patronising. Don't assume I know gym terminology. Use British English throughout. And please don't tell me to "consult my GP first" as a disclaimer — instead, tell me specifically which of my symptoms actually would be worth mentioning to a doctor.