Emily, my shorts are definitely not too small... they are x smalls and I've been about the same size since I started cycling, well, at least once i got a road bike. I can't explain it, except that even the most minute differences in geometry bother me and I just feel more comfortable and my skills are all much better on my Kuota. So much for custom. I think for me, if I'm used to it, that's it. So while the Kuota may be a bit scrunched for me, I *like* that feel because I am used to it. Once I am riding on the Guru, I'm fine, and I like the feel of the titanium.
As far as the age thing, I'm 60, but I have always needed more rest than others, it seems. I don't know how some of the people in my groups do it, except perhaps, that riding is all they do! On the other hand, when I've done tours, I can suck it up and do consecutive 40-60 mile days. I could not do 100 in the mix, at least not now, without a little more training. It is mental, to a large degree, and the amount of suffering you are willing to endure. All I know, is that I felt pretty good on my hot and fast for me 42 mile ride yesterday, because I did nothing on Tuesday.