Re: training troubles
I think Ron's advice is spot on - if you don't actually feel unsafe being his uke, then use what he is doing as nage to improve your own aikido by staying ahead of him - having said that, I don't think that, when I was a 5th kyu, I would have been able to do this! : neither my understanding of the idealized form of the technique nor my ability to be that well connected to nage would have been up to it.
One thing you, or anybody, can do is very nicely smile & explain you are having a little trouble keeping up, could he please slow down? (I do understand that the issue you identify is muscle, not speed, but this way you infer the fault is your's for being a dumb 5th kyu AND if he is able to slow down, even if he is muscling, you'll be more likely to be able to breathe, center yourself, and work on the skills you need to follow Ron's advice.)
My 2 cents.