Any advice on tackling a BIG cleaning job?

Just be patient with yourself.
Instead of doing two hours and ending up exhausted,
just do one and a half per day.

Give yourself a set of priorities, and aim to accomplish one each day - but if it's not done within your time limit, just finish it the next.

Don't try to spring clean. Just do the basics at first until things are restored to a relatively acceptable norm.

Get out for a little bit of gentle exercise each day. Just walking in a nice place would be fine. Only do as much as is comfortable, but make it last five minutes longer each day.

For instance
1. Check bills and pay, or arrange a payment schedule and reminder system.
2. any old food in the fridge goes to the compost, worms or chooks, dirty dishes rinsed and stacked ready for washing, dirty clothes and linen chucked in the laundry, sort and put out the rubbish bins.
3. wet areas: (might need one or more sessions for each) kitchen, laundry, toilet and bathroom.
If the whole is too much, just do the most important bits and come back to the rest on another day.
4. dry tasks: go from top to bottom (because dirts falls), cobwebs, dusting, sweeping or vacuuming, mopping if necessary. One room at a time. Choose the room you like to spend most time in to do first.

Once all those basics are done, compile your list of spring cleaning and sorting tasks. Keep it small, one cupboard or minor task per day. If the fridge is to big, do one shelf at a time.

Give yourself a reward for every task done - like relaxing with music, book or movie, time on the phone with a friend, a long bath, a luscious peach, burning a stick of incense.

One tricky thing is dealing with frequently reoccuring things like clothes washing and dishes. Try not to spend too much time or focus on those becuase they will prevent you from moving onto the less occuring things. Once you get the less occuring things done first, you can go all out on the ones that happen much more frequently.

What look like huge projects can be whittled down to manageable tasks.

1) concentrate on clutter that is actually trash - junk mail, paper, trash. Bag it in bags small enough for you to carry to the dumpster or whatever.

2) Separate the good stuff left - clothes, etc. and bag it too (don't let it get confused with the real junk that you're pitching out).

3) think about asking a housemate to carry out a bag of your trash when they're taking theirs.

4) If there's a senior resource center in your area, call and ask for help. A church or a charity may have volunteers who'd pitch in and help you get back to normal.

I think the first thing is to build up your strength again. first focus on not letting it get worse, then dedicate some portions of your day to cleaning up more and make sure you get lots of break time in between sessions.
it seems like it might help your mental well being to focus on one particular area and get that cleaned up, then move to the next area.

I found a cleaning service on craigslist that charged only 100$ for a couple hours and instructed them to specifically do the major bulk of it which really helped me out making it not so overwhelming.

Only other suggestion is to just start and make small steps little at a time.

The only way is to pace yourself. Chip away at it a little at a time and break it down into smaller, more manageable tasks between which you can rest.

It also helps me to just accept it will take me thrice as long to complete the task as any other person. So I waste less time worrying about it and spend more time doing it.

I wish you had some help and I'm sorry that you don't. 🤗 I have the mind to slap anyone who ever dared to call you senile.

Slow and steady wins the race.Do a bit at a time.
Do what you can do.
Do what is possible.
Before you know it, you'll be doing the impossible!!

Sorry to say but basically, you're fucked