Slightly worried about giving training advice to someone recovering from a bout of ME. However the principles are quite simple, either shorter and faster/harder than your normal ride time, or ride slower and longer.
If your standard ride with your mates is a couple of hours then ride for longer at a slower tempo sometimes, but more importantly for people in real world, (time commitments), ride for a hour faster/harder. Either as intervals, (blast the hills), or at a solid tempo you know you couldn’t hold for more than an hour.