I've been there in rebuilding fitness and muscle after wastage. In my case it was from hard pregnancies with nausea then SPD which killed my appetite and energy, then ended up being housebound because of pelvic pain. Add in a EMCS and complications in HDU to recover from too.
After the quick birth weight had gone, despite being 2st heavier than usual, by legs were thinner than usual because I'd barely used them for 9+months.
I started with using walking poles to walk 100m x2 to the end of the road and back (and went to bed to recover) and gradually extended a bit more each day. At 6 weeks I could add in post-natal pilates. After 2 months I could walk a couple of miles and could go to a gentle post-natal class. At 5 months I was fit enough to go to a mainstream class. Logistically the best one was a somewhat macho circuits class and it was before the culture of women lifting too had really got going, but with hindsight it was my best fitness move because it introduced me to weights.
I had to repeat the process two years later after another baby.
That time I phased in C25k as I was fed up of huffing and puffing everywhere, and less than 18m after hobbling around on crutches I ran a half marathon. (I loathed running at school, but it's much nicer without a bellowing PE teacher and better scenery than loops of a school field)
With a plan to gradually progress what you do, getting fitter and stronger is absolutely doable.*
It's not easy pushing yourself to do it, but life being unfit and weak isn't easy either. This is why I'm such a strong advocate for exercise and functional fitness. I also want to do my best not to suffer 20+ years of osteoporosis in later life like other generations of my family.
Being strong means little everyday
activities get easier for example instead of having to battle for a busy parking space near the supermarket and rattle the shopping trolley backwards and forwards across the car park, I can be "lazy" pick an easy parking space at the back and shove the food shopping into two IKEA bags, stick one on each shoulder, park the trolley up and do a single carry to the car (and my teenagers get through a lot of heavy cans of rice pudding!)
- You can do this DIY from home, but you will need to do your research on finding a plan to progress you or it's easy to get stuck doing random videos and not progressing. It's also very difficult for checking for good form on your own especially if there are long term weaknesses/ imbalances. This is why PTs, gyms and classes are valuable to novices.