I've ridden a lot of long randonnees on steel frames with fairly relaxed angles and generous fork rake. Good gloves, shoes and/or bar padding will help, as will a saddle you really get along with. Spend the time and money, if necessary, to ensure the best position on the bike, even if this means going to the trouble and expense of custom fitting.

Compared with the kind of frame you might look at for criteriums or short time trials, my Randonnee bike looks like a tourer; there's a good reason for that,- you ain't going far if you can't sit comfortably. If you've had problems with your joints (and they were caused or exacerbated by riding the bike?)you might wish to follow a similar path, rather than adapting a stiff race frame, say,- then having to buy all kinds of gadgets to make it more comfortable.

You already know that weight is a red herring. I could skim bits off the weight of my bike, at the risk of decreased durability but when you are riding all day- or all weekend- you need to carry stuff and most importantly, you need to cushion your body somewhat.