I agree the distance - speed - time thing is the huge challenge for technology and biological life forms like humans. Various ways round may be found - ways of increasing the speed, cryo- technology, or autonomous ways of 'parenting' egg/sperm cells, once mechanised colonisers have found and prepared the right conditions. Of course, all that is speculation. As AI grows more powerful, it's possible that solutions will start to develop exponentially. I tend to think, for the very issue you raise about the weak link of human biology, that a major part of space colonisation may involve autonomous mechanised AI making the pioneer journeys, building the initial structures etc. You could argue, at that point, that the artificial intelligence wouldn't even need humans anymore, but there's also the prospect of human/AI interfaces or uploading.
I do believe in the future of colonisation though. It's deep in the human DNA. The only thing that could stop us in the long-term, I think, is if we self-destruct. I don't think we will, but there is a possibility that wealth divides will widen, and space will become the domain of the huge corporations and incredibly rich, not to mention the possibilities of those kind of entities seceding from the rest of the species.
The voyager missions, using such basic computer technology, are heroic really. Still ploughing their way towards inter-stellar vastness, epitomising adventure and hope.