In reply to RJNewbie:
Congratulations, on reaching your first summit and in wanting to extend the challenge.
You will find a lot of people on here who don't like the 3 peaks challenge, people see it more of a driving challenge, they don't like the fact that it involves disturbing the people of Wasdale early in the morning, and they don't like the commercial aspect of it and the large groups of people who do it so be prepared for that!
Have you considered climbing other hills in preparation for your next challenge, there are literally hundreds of hills you can walk up in the UK and they can be linked together to give a hugely challenging day out without driving hundreds of miles, you might find that you are happy challenging yourself on your local hills and save the big 3 for when you are ready to do them independently? If so, where do you live? Are you near any hills, if so people could suggest routes which are suitable for beginners.
Altitude sickness isn't a problem on british hills, and wouldn't be a problem the next day so I guess your friend was sick with something else.
Downhill - walk down big hills and you get better at it, no secrets really! You could try running when you are strong enough, as some people find it is less jarring on the knees/muscles but don't run before you can walk!