Transverse Myelitis and Exhaustion

OK. I am semi-retired. 
So I have been back at work for two days (Wednesday and Thursday). That means we drive for about an hour in motorway conditions to get to work. Unloading the car, we walk carefully down a small slope to the office. Then I work from 9am to 3pm with 30 mins for lunch. Then we walk back up the short hill and get back on the road for home. I have managed my caseload well, feel energised by the work that I do, have a sense of having a meaningful role in helping others. But by the second day I really have had enough.
I now know from my clever little iPhone, that I take a total of about 1700 steps on work days. But in contrast, when we are home I move around the place, and then try deliberately to take a walk of at least that length a couple of days a week. So no different.
So what is making the difference? I don't think it is working with people - I have been committed to doing that for 50 years, and love it. I don't think it is the people at work - they are delightful. I eat reasonably well, even though my TM has led to considerable muscle loss, which means I am 15 Kilos lighter than my fighting weight. Air conditioning - I don't think so.
It may be that I do not drink enough during work days - there is just not enough time between patients to manage frequent visits to the toilet. So often when I get home I try to make up for this in the evening.
But, by Fridays, I feel like doing little but sitting, and doing work (or having fun) on the computer... I feel tired, my legs do not work or walk as well as usual, and it takes till Saturday for me to feel enthusiastic enough to want to walk or swim or go briefly on the indoor bike, or meet family and friends. Just nothing seems to make much difference. We have tried changes in diet, shorter days, sharing driving with my long-suffering partner - it is good but does not make a difference.
My back aches in the usual old place, and the referred pain around my right chest is always worse. I have tried various sitting positions, added cushions, taken breaks, and nothing makes much difference. I don't like taking medications. But on Fridays I often do take a couple of paracetomol, which takes the edge off. I refuse to take anything stronger, preferring to use meditation and acupuncture points to relieve acute pain.
So today is Friday. I just feel exhausted.

Damned transverse myelitis. Five years and not remitting...

At least I CAN walk (sort of).
But at least I CAN work.
I count myself as lucky.
Tomorrow I will push for a bit of exercise. Perhaps we can have a short swim later today...