It sounds like you just overdid it a bit to me. I too am having an exhausted day and feeling a bit frustrated by it. Although I am only mildly affected by MS, on days like today it is much more extreme. Fatigue, muscle aches in my limbs, crampy style tightness in my calves, more obvious numbness in my right hand and leg, foot drop and weakness as soon as I start to walk instead of after a mile or so. All a bit tiring and annoying as I've had such a good week.
That's when I find it happens.
I think I get a bit over zealous because I feel good and then a few days later it hits me that I've done too much. On days like today I do an extra long meditation in a restful position (lying down for me), make sure I eat something warm and nourishing like homemade soup and bread, or savoury rice with ginger and chilli, and just get as much rest as possible. Once the worst of it passes I start to introduce some gentle yoga, focusing on my breath and opening out my chest and ribs area to increase oxygen intake whilst I breathe.
I find this really quickly helps me get back to where I was, and actually the breathing and yoga is something I try and do every day in addition to meditation. It's actually when I haven't done it for a few days due to busy-ness, that my fatigue appears.
And camomile tea!!! I drink gallons of it when I'm feeling exhausted. It helps me relax, sleep well and feel less "inflamed"!!!
Hope this helps.
I am a yoga teacher, so if you want any postures/breathing exercises I would be happy to send you some on here
Well done on the cycling and walking btw. If love to hear about your progress. I'm hoping to restart my running again soon (2/3 mins at a time of course!!)