Managing post exertional malaise

driving is a big deal for me. I haven't driven more than 1 1/2 hours since November 2020. We don't realize how much 'brain work' is involved. If I am going to do something taxing I plan nothing the day before and after. Normally I cross out 2 days a week for recovery; no appointments, errands, detailed/complicated brain work. It's what I need to be able to function the other 5 days a week. If I don't feel well I cancel my plans for the day.
It's a yo-yo life and taken 3.5 years to find my balance and where my boundaries are. There are far less things I really HAVE to do than I thought-sometimes I WANT so much to do something that it feels like a need, but it really isn't. Prioritizing, pacing and pivoting are my key words. Your balance is not the same as anyone else's. The longest I have gone without a crash is 4 weeks-I just keep forgetting to manage myself and revert to pre-covid expectations.
PT, OT, Speech made a big difference in figuring out how to do things differently.

At the risk of being redundant, I crashed after doing a 20 minute walk video. This was my first, and I need help. What do you do all day while recovering?
My crash is a feeling of weakness, deeper fatigue. I am able to do the same things but I just feel like moving slower. Any suggestions would be so appreciated.

This may sound strange, but a lot of covid weakness is caused by a low amount of oxygen getting through the arteries / blood vessels that are shrunk tight because of low amount of nitric oxide. You should have foods with nitrates that the body uses to make nitric oxide to allow the arteries to open up.
They say the best way to increase nitric oxide is to have dark greens. The darker the better. When we chew the greens, the good bacteria in our mouth creates the nitric oxide. (Remember, no mouthwash or toothpaste with fluoride that could knock out the good bacteria.)
I try for a type of curly Italian kale, like dinosaur or Lucinato / Tuscan that looks tough but is tender. ( Other choices are arugula or spinach. ) I rinse the kale well and than pull it off the stems and cut it up with kitchen shears.
Beets are a great source of nitrate and you could put some on the greens.
Blueberries are good and could also be added to the salad or have by themselves.
Watermelon is a good source.
If you could find unsweetened cocoa and add honey. I put it in decaf coffee, but you could add or use milk or almond milk instead.
Nuts and celery are a good source, so a nut butter on celery sticks would be good.
Also include some orange slices for vitamin C which is an antioxidant that helps with nitric oxide production.

Think of your body as a battery that only has so much charge in one day. Gauge your daily schedule accordingly. Easy for me to say and not easy to do - another way of describing pacing. Don't forget that mental and physical activities drain your battery, too.

For muscle cramps / spasms I use potassium and magnesium.

"Potassium is a nutrient that helps facilitate muscle contractions. It is a neuromuscular transmitter that provides communication between muscles and nerves. This communication breaks down when potassium levels are low, and muscles can “get stuck” in a contracted position that we feel as spasms or cramps. Historically, potassium has been associated with bananas. However, they are not the best nutrient source, providing just nine percent of your daily recommended intake. Instead, consider one of the following five foods listed below to help with muscle cramps:

Sweet potatoes
Melon
Cooked Spinach
Nuts
Beans"
https://www.wdhospital.org/wdh/services-and-specialties/orthopedic-care/blog/nutrition-tips-to-prevent-cramps#:~:text=Potassium%20is%20a%20nutrient%20that,feel%20as%20spasms%20or%20cramps.
I use "No Salt" for seasoning which is 100% potassium. It is in most groceries, made in Germany. There is also "light salt" that is 50-50 sodium / potassium.
I also take magnesium taurate daily. With those two, I have eliminated the cramps. Hope it can help you.

I can’t let myself get excited—the excitement for me can cause PEM just as easily as too much physical effort.

LC is not like training for a 5K run, it is more like being run over by a train after a 5K walk. I got covid last March on a 7-day 350 bike tour. I normally bike ~7000 mi/year. The LC hit after covid, and I went from biking up to 80 mi/day to 5 mi on an exercise bike with a nap afterwards. My LC went away after 8 months. LC hits endurance and once you are depleted, the recovery is much slower than before. So, I would exercise in snippets (5 mi), rest for a while, then bike or walk a bit, and rest, and all at levels that would not trigger the LC fatigue (also termed PEM). On a happier note, LC doesn't seem to hit resistance training, so if your husband is fine with you lifting weights or other (nonaerobic) exercise then you can still keep your muscle mass and bone density until you are over LC. Make sure you get enough sleep and avoid stress; those hit LC as well. Food intolerances can be triggered too, so consider keeping a food log.