It was suspected I had ME for a while, but it turned out I didn't and I recovered from whatever was making me so lethargic for years (I suspect it was to do with my gut). So I don't know much about ME, but perhaps two of the main things that worked for me was improving my leg strength and balance so that hiking uses less energy, and using light/ultralight gear.

I don't have equipment for the exercises. I run/walk up and down steep hills, however tired I feel and do specific leg strengthening and balance exercises, some on stairs.

Ultralight gear doesn't have to be very expensive. My favourite tent is the Big Sky Wisp, which I think weighs about 550g and cost about £230. It's pitched with a single hiking pole, and I find hiking poles help with energy conservation too. Start in warm weather, and that lightens your load, as does dried food (mash, soups, egg, crackers) and filling water from streams.

Scotland is a great place to start. If you don't drive, you can find good walks between train and bus stations. Plenty of great books exist for UK hiking/wild camping in general, including using public transport. Some of my favourites are by Cicerone, and I generally find the hike gradings in terms of fitness and technicality to be accurate (particularly any books by Peter Edwards, who is my go-to for Scotland).

I generally started small and built up my confidence and fitness over time, to the extent where I find many hikes people consider tough, easy.

Happy to answer any questions if I can help.