If you're indeed riding at 40 to 50k at 40-45kph, there should be a number of domestic or local pro teams knocking at your door. Riding a 40k TT in under an hour is very difficult for most people. If you can ride 80k in under 2 hours, get on a plane and head to Europe. There's a career in there somewhere for you.

Yes, riding slow is more painful, but did you tell your friend what was going on?

"Man, I'm really sorry, but I'm hurting like hell and need to get back by ?:00 pm...do you mind if I take off?"

"Sure, no problem"

" Are you good to go for the ride back alone?"

"Yes"

"Need extra water?"

"No, I'm good."

"Alright, I'll give you a call later."

"no worries"

...courtesy, eh? If that's what you did, then "dropping" you friend was fine. (If it was my friend, I'd say, "ride on ahead home." ) If not, there are some other issues going on. As a friend and more experienced cyclist, did you evaluate if he was bonking, hypothermic, dehydrated and maybe had too much pride to tell you? Sorry to be a nag, but many of us have been exactly in this situation. Things can go south quickly and sometimes dangerously if a rider is beyond his/her limit for a long period of time without either of you being aware of it.