//" So maybe first I'll just work up to him being a very athletic, strong dog, and then try a backpack MAYBE"//
good plan!
remember, there are experts who think, that dogs under 18 mos old should not walk or run too far nonstop.
It's one thing if a dog walks or runs by himself, for a few yards here,
or a few yards there, stopping every few minutes.
It's a whole other thing to tie an immature dog to a bike and go for miles.
See, some ppl think the repetitive movements or certain moves/positions if held very long,
like standing on any two paws only, jumping from more than the dog's shoulder height, (or less, depending on age of baby dog)
running, nonstop long walks,
might stress the still-soft skeleton of an immature dog, (under 18 to 24 mos old)
See, their concern is,
that nonstop repetitive moves could stress the still-soft joints of the dog,
and possibly increase the risk of joint problems as an older dog.